python pyqt pyqt5 relative-path qtstylesheets

Using relative paths in Qt stylesheet urls

I have trouble reading the images in a .qss file. The organizations is as following:

How can I fix this bug ?
Is there a proper way to append all folders/subfolder to the path of the main application main.py. I have tried sys.path.append('./images'), but it always requires the absolute path to any image or icon ?

Example Code (Note that No icons are appearing (no down_arrow for QCombobox, and no question icon for messagebox)):

import sys
from PyQt5.QtWidgets import *
from PyQt5.QtGui import *
from PyQt5.QtCore import *

class Example(QWidget):
    def __init__(self):
        super().__init__()
        
        self.initUI()
        
    def initUI(self):       
        self.hbox  = QHBoxLayout()
        self.setLayout(self.hbox)
        self.combo = QComboBox()
        self.combo.addItem('item_1')
        self.combo.addItem('item_2')
        self.combo.setCurrentText('item_1')
        self.hbox.addWidget(self.combo)
        
        self.bt = QPushButton('Question')
        self.hbox.addWidget(self.bt)
        self.bt.clicked.connect(self.openMess)

        # style
        file       = QFile(r'./themes/DarkBlue.qss')
        file.open(QFile.ReadOnly | QFile.Text)
        stylesheet = file.readAll()
        app.setStyleSheet(str(stylesheet, encoding='utf-8'))

    def openMess(self):
        buttonReply    = QMessageBox.question(self, 'No Image', "Check icons", QMessageBox.Yes | QMessageBox.No, QMessageBox.No)
        if buttonReply == QMessageBox.Yes:
            print('Yup')
        else:
            print('Nup')


if __name__ == '__main__':
    app = QApplication(sys.argv)
    ex = Example()
    ex.show()
    sys.exit(app.exec_())

This is a sample of the .qss file i use. The images folder is a subfolder at same location as the .qss file:

QMenuBar
{
    background-color: #302F2F;
    color: silver;
}

QMenuBar::item
{
    background: transparent;
}

QMenuBar::item:selected
{
    background: transparent;
    border: 1px solid #3A3939;
}

QMenuBar::item:pressed
{
    border: 1px solid #3A3939;
    background-color: #3d8ec9;
    color: black;
    margin-bottom:-1px;
    padding-bottom:1px;
}

QMenu
{
    border: 1px solid #3A3939;
    color: silver;
    margin: 1px;
}

QMenu::icon
{
    margin: 1px;
}

QMenu::item
{
    padding: 2px 2px 2px 25px;
    margin-left: 5px;
    border: 1px solid transparent; /* reserve space for selection border */
}

QMenu::item:selected
{
    color: black;
}

QMenu::separator {
    height: 2px;
    background: lightblue;
    margin-left: 10px;
    margin-right: 5px;
}

QMenu::indicator {
    width: 16px;
    height: 16px;
}

/* non-exclusive indicator = check box style indicator
   (see QActionGroup::setExclusive) */
QMenu::indicator:non-exclusive:unchecked {
    image: url(images/checkbox_unchecked.png);
}

QMenu::indicator:non-exclusive:unchecked:selected {
    image: url(images/checkbox_unchecked_disabled.png);
}

QMenu::indicator:non-exclusive:checked {
    image: url(images/checkbox_checked.png);
}

QMenu::indicator:non-exclusive:checked:selected {
    image: url(images/checkbox_checked_disabled.png);
}

/* exclusive indicator = radio button style indicator (see QActionGroup::setExclusive) */
QMenu::indicator:exclusive:unchecked {
    image: url(images/radio_unchecked.png);
}

QMenu::indicator:exclusive:unchecked:selected {
    image: url(images/radio_unchecked_disabled.png);
}

QMenu::indicator:exclusive:checked {
    image: url(images/radio_checked.png);
}

QMenu::indicator:exclusive:checked:selected {
    image: url(images/radio_checked_disabled.png);
}

QMenu::right-arrow {
    margin: 5px;
    image: url(images/right_arrow.png)
}


QWidget:disabled
{
    color: #808080;
    background-color: #302F2F;
}

QAbstractItemView
{
    alternate-background-color: #3A3939;
    color: silver;
    border: 1px solid 3A3939;
    border-radius: 2px;
    padding: 1px;
}

QWidget:focus, QMenuBar:focus
{
    border: 1px solid #78879b;
}

QTabWidget:focus, QCheckBox:focus, QRadioButton:focus, QSlider:focus
{
    border: none;
}

QLineEdit
{
    background-color: #201F1F;
    padding: 2px;
    border-style: solid;
    border: 1px solid #3A3939;
    border-radius: 2px;
    color: silver;
}

QGroupBox {
    border:1px solid #3A3939;
    border-radius: 2px;
    margin-top: 20px;
    background-color: #302F2F;
    color: silver;
}

QGroupBox::title {
    subcontrol-origin: margin;
    subcontrol-position: top center;
    padding-left: 10px;
    padding-right: 10px;
    padding-top: 10px;
}

QAbstractScrollArea
{
    border-radius: 2px;
    border: 1px solid #3A3939;
    background-color: transparent;
}

QScrollBar:horizontal
{
    height: 15px;
    margin: 3px 15px 3px 15px;
    border: 1px transparent #2A2929;
    border-radius: 4px;
    background-color: #2A2929;
}

QScrollBar::handle:horizontal
{
    background-color: #605F5F;
    min-width: 5px;
    border-radius: 4px;
}

QScrollBar::add-line:horizontal
{
    margin: 0px 3px 0px 3px;
    border-image: url(images/right_arrow_disabled.png);
    width: 10px;
    height: 10px;
    subcontrol-position: right;
    subcontrol-origin: margin;
}

QScrollBar::sub-line:horizontal
{
    margin: 0px 3px 0px 3px;
    border-image: url(images/left_arrow_disabled.png);
    height: 10px;
    width: 10px;
    subcontrol-position: left;
    subcontrol-origin: margin;
}

QScrollBar::add-line:horizontal:hover,QScrollBar::add-line:horizontal:on
{
    border-image: url(images/right_arrow.png);
    height: 10px;
    width: 10px;
    subcontrol-position: right;
    subcontrol-origin: margin;
}


QScrollBar::sub-line:horizontal:hover, QScrollBar::sub-line:horizontal:on
{
    border-image: url(images/left_arrow.png);
    height: 10px;
    width: 10px;
    subcontrol-position: left;
    subcontrol-origin: margin;
}

QScrollBar::up-arrow:horizontal, QScrollBar::down-arrow:horizontal
{
    background: none;
}


QScrollBar::add-page:horizontal, QScrollBar::sub-page:horizontal
{
    background: none;
}

QScrollBar:vertical
{
    background-color: #2A2929;
    width: 15px;
    margin: 15px 3px 15px 3px;
    border: 1px transparent #2A2929;
    border-radius: 4px;
}

QScrollBar::handle:vertical
{
    background-color: #605F5F;
    min-height: 5px;
    border-radius: 4px;
}

QScrollBar::sub-line:vertical
{
    margin: 3px 0px 3px 0px;
    border-image: url(images/up_arrow_disabled.png);
    height: 10px;
    width: 10px;
    subcontrol-position: top;
    subcontrol-origin: margin;
}

QScrollBar::add-line:vertical
{
    margin: 3px 0px 3px 0px;
    border-image: url(images/down_arrow_disabled.png);
    height: 10px;
    width: 10px;
    subcontrol-position: bottom;
    subcontrol-origin: margin;
}

QScrollBar::sub-line:vertical:hover,QScrollBar::sub-line:vertical:on
{

    border-image: url(images/up_arrow.png);
    height: 10px;
    width: 10px;
    subcontrol-position: top;
    subcontrol-origin: margin;
}


QScrollBar::add-line:vertical:hover, QScrollBar::add-line:vertical:on
{
    border-image: url(images/down_arrow.png);
    height: 10px;
    width: 10px;
    subcontrol-position: bottom;
    subcontrol-origin: margin;
}

QScrollBar::up-arrow:vertical, QScrollBar::down-arrow:vertical
{
    background: none;
}


QScrollBar::add-page:vertical, QScrollBar::sub-page:vertical
{
    background: none;
}

QTextEdit
{
    background-color: #201F1F;
    color: silver;
    border: 1px solid #3A3939;
}

QPlainTextEdit
{
    background-color: #201F1F;;
    color: silver;
    border-radius: 2px;
    border: 1px solid #3A3939;
}

QHeaderView::section
{
    background-color: #3A3939;
    color: silver;
    padding-left: 4px;
    border: 1px solid #6c6c6c;
}

QSizeGrip {
    image: url(images/sizegrip.png);
    width: 12px;
    height: 12px;
}

QMainWindow
{
    background-color: #302F2F;

}

QMainWindow::separator
{
    background-color: #302F2F;
    color: white;
    padding-left: 4px;
    spacing: 2px;
    border: 1px dashed #3A3939;
}

QMainWindow::separator:hover
{

    background-color: #787876;
    color: white;
    padding-left: 4px;
    border: 1px solid #3A3939;
    spacing: 2px;
}


QMenu::separator
{
    height: 1px;
    background-color: #3A3939;
    color: white;
    padding-left: 4px;
    margin-left: 10px;
    margin-right: 5px;
}


QFrame
{
    border-radius: 2px;
    border: 1px solid #444;
}

QFrame[frameShape="0"]
{
    border-radius: 2px;
    border: 1px transparent #444;
}

QStackedWidget
{
    background-color: #302F2F;
    border: 1px transparent black;
}

QToolBar {
    border: 1px transparent #393838;
    background: 1px solid #302F2F;
    font-weight: bold;
}

QToolBar::handle:horizontal {
    image: url(images/Hmovetoolbar.png);
}
QToolBar::handle:vertical {
    image: url(images/Vmovetoolbar.png);
}
QToolBar::separator:horizontal {
    image: url(images/Hsepartoolbar.png);
}
QToolBar::separator:vertical {
    image: url(images/Vsepartoolbars.png);
}

QPushButton
{
    color: silver;
    background-color: #302F2F;
    border-width: 2px;
    border-color: #4A4949;
    border-style: solid;
    padding-top: 2px;
    padding-bottom: 2px;
    padding-left: 10px;
    padding-right: 10px;
    border-radius: 4px;
    /* outline: none; */
    /* min-width: 40px; */
}

QPushButton:disabled
{
    background-color: #302F2F;
    border-width: 2px;
    border-color: #3A3939;
    border-style: solid;
    padding-top: 2px;
    padding-bottom: 2px;
    padding-left: 10px;
    padding-right: 10px;
    /*border-radius: 2px;*/
    color: #808080;
}

QPushButton:focus {
    background-color: #3d8ec9;
    color: white;
}

QComboBox
{
    selection-background-color: #3d8ec9;
    background-color: #201F1F;
    border-style: solid;
    border: 1px solid #3A3939;
    border-radius: 2px;
    padding: 2px;
    min-width: 75px;
}

QPushButton:checked{
    background-color: #4A4949;
    border-color: #6A6969;
}

QPushButton:hover {
    border: 2px solid #78879b;
    color: silver;
}

QComboBox:hover, QAbstractSpinBox:hover,QLineEdit:hover,QTextEdit:hover,QPlainTextEdit:hover,QAbstractView:hover,QTreeView:hover
{
    border: 1px solid #78879b;
    color: silver;
}

QComboBox:on
{
    background-color: #626873;
    padding-top: 3px;
    padding-left: 4px;
    selection-background-color: #4a4a4a;
}

QComboBox QAbstractItemView
{
    background-color: #201F1F;
    border-radius: 2px;
    border: 1px solid #444;
    selection-background-color: #3d8ec9;
    color: silver;
}

QComboBox::drop-down
{
    subcontrol-origin: padding;
    subcontrol-position: top right;
    width: 15px;

    border-left-width: 0px;
    border-left-color: darkgray;
    border-left-style: solid;
    border-top-right-radius: 3px;
    border-bottom-right-radius: 3px;
}

QComboBox::down-arrow
{
    image: url(images/down_arrow_disabled.png);
}

QComboBox::down-arrow:on, QComboBox::down-arrow:hover,
QComboBox::down-arrow:focus
{
    image: url(images/down_arrow.png);
}

QPushButton:pressed
{
    background-color: #484846;
}

QAbstractSpinBox {
    padding-top: 2px;
    padding-bottom: 2px;
    border: 1px solid #3A3939;
    background-color: #201F1F;
    color: silver;
    border-radius: 2px;
    min-width: 75px;
}

QAbstractSpinBox:up-button
{
    background-color: transparent;
    subcontrol-origin: border;
    subcontrol-position: top right;
}

QAbstractSpinBox:down-button
{
    background-color: transparent;
    subcontrol-origin: border;
    subcontrol-position: bottom right;
}

QAbstractSpinBox::up-arrow,QAbstractSpinBox::up-arrow:disabled,QAbstractSpinBox::up-arrow:off {
    image: url(images/up_arrow_disabled.png);
    width: 10px;
    height: 10px;
}
QAbstractSpinBox::up-arrow:hover
{
    image: url(images/up_arrow.png);
}


QAbstractSpinBox::down-arrow,QAbstractSpinBox::down-arrow:disabled,QAbstractSpinBox::down-arrow:off
{
    image: url(images/down_arrow_disabled.png);
    width: 10px;
    height: 10px;
}
QAbstractSpinBox::down-arrow:hover
{
    image: url(images/down_arrow.png);
}


QLabel
{
    border: 0px solid black;
}

QTabWidget{
    border: 1px transparent black;
}

QTabWidget::pane {
    border: 1px solid #444;
    border-radius: 3px;
    padding: 3px;
}

QTabBar
{
    qproperty-drawBase: 0;
    left: 5px; /* move to the right by 5px */
}

QTabBar:focus
{
    border: 0px transparent black;
}

QTabBar::close-button  {
    image: url(images/close.png);
    background: transparent;
}

QTabBar::close-button:hover
{
    image: url(images/close-hover.png);
    background: transparent;
}

QTabBar::close-button:pressed {
    image: url(images/close-pressed.png);
    background: transparent;
}

/* TOP TABS */
QTabBar::tab:top {
    color: #b1b1b1;
    border: 1px solid #4A4949;
    border-bottom: 1px transparent black;
    background-color: #302F2F;
    padding: 5px;
    border-top-left-radius: 2px;
    border-top-right-radius: 2px;
}

QTabBar::tab:top:!selected
{
    color: #b1b1b1;
    background-color: #201F1F;
    border: 1px transparent #4A4949;
    border-bottom: 1px transparent #4A4949;
    border-top-left-radius: 0px;
    border-top-right-radius: 0px;
}

QTabBar::tab:top:!selected:hover {
    background-color: #48576b;
}

/* BOTTOM TABS */
QTabBar::tab:bottom {
    color: #b1b1b1;
    border: 1px solid #4A4949;
    border-top: 1px transparent black;
    background-color: #302F2F;
    padding: 5px;
    border-bottom-left-radius: 2px;
    border-bottom-right-radius: 2px;
}

QTabBar::tab:bottom:!selected
{
    color: #b1b1b1;
    background-color: #201F1F;
    border: 1px transparent #4A4949;
    border-top: 1px transparent #4A4949;
    border-bottom-left-radius: 0px;
    border-bottom-right-radius: 0px;
}

QTabBar::tab:bottom:!selected:hover {
    background-color: #78879b;
}

/* LEFT TABS */
QTabBar::tab:left {
    color: #b1b1b1;
    border: 1px solid #4A4949;
    border-left: 1px transparent black;
    background-color: #302F2F;
    padding: 5px;
    border-top-right-radius: 2px;
    border-bottom-right-radius: 2px;
}

QTabBar::tab:left:!selected
{
    color: #b1b1b1;
    background-color: #201F1F;
    border: 1px transparent #4A4949;
    border-right: 1px transparent #4A4949;
    border-top-right-radius: 0px;
    border-bottom-right-radius: 0px;
}

QTabBar::tab:left:!selected:hover {
    background-color: #48576b;
}


/* RIGHT TABS */
QTabBar::tab:right {
    color: #b1b1b1;
    border: 1px solid #4A4949;
    border-right: 1px transparent black;
    background-color: #302F2F;
    padding: 5px;
    border-top-left-radius: 2px;
    border-bottom-left-radius: 2px;
}

QTabBar::tab:right:!selected
{
    color: #b1b1b1;
    background-color: #201F1F;
    border: 1px transparent #4A4949;
    border-right: 1px transparent #4A4949;
    border-top-left-radius: 0px;
    border-bottom-left-radius: 0px;
}

QTabBar::tab:right:!selected:hover {
    background-color: #48576b;
}

QTabBar QToolButton::right-arrow:enabled {
     image: url(images/right_arrow.png);
 }

 QTabBar QToolButton::left-arrow:enabled {
     image: url(images/left_arrow.png);
 }

QTabBar QToolButton::right-arrow:disabled {
     image: url(images/right_arrow_disabled.png);
 }

 QTabBar QToolButton::left-arrow:disabled {
     image: url(images/left_arrow_disabled.png);
 }


QDockWidget {
    border: 1px solid #403F3F;
    titlebar-close-icon: url(images/close.png);
    titlebar-normal-icon: url(images/undock.png);
}

QDockWidget::close-button, QDockWidget::float-button {
    border: 1px solid transparent;
    border-radius: 2px;
    background: transparent;
}

QDockWidget::close-button:hover, QDockWidget::float-button:hover {
    background: rgba(255, 255, 255, 10);
}

QDockWidget::close-button:pressed, QDockWidget::float-button:pressed {
    padding: 1px -1px -1px 1px;
    background: rgba(255, 255, 255, 10);
}

QTreeView, QListView, QTextBrowser, AtLineEdit, AtLineEdit::hover {
    border: 1px solid #444;
    background-color: silver;
    border-radius: 3px;
    margin-left: 3px;
    color: black;
}

QTreeView:branch:selected, QTreeView:branch:hover {
    background: url(images/transparent.png);
}

QTreeView::branch:has-siblings:!adjoins-item {
    border-image: url(images/transparent.png);
}

QTreeView::branch:has-siblings:adjoins-item {
    border-image: url(images/transparent.png);
}

QTreeView::branch:!has-children:!has-siblings:adjoins-item {
    border-image: url(images/transparent.png);
}

QTreeView::branch:has-children:!has-siblings:closed,
QTreeView::branch:closed:has-children:has-siblings {
    image: url(images/branch_closed.png);
}

QTreeView::branch:open:has-children:!has-siblings,
QTreeView::branch:open:has-children:has-siblings  {
    image: url(images/branch_open.png);
}

QTreeView::branch:has-children:!has-siblings:closed:hover,
QTreeView::branch:closed:has-children:has-siblings:hover {
    image: url(images/branch_closed-on.png);
    }

QTreeView::branch:open:has-children:!has-siblings:hover,
QTreeView::branch:open:has-children:has-siblings:hover  {
    image: url(images/branch_open-on.png);
    }

QListView::item:!selected:hover, QListView::item:!selected:hover, QTreeView::item:!selected:hover  {
    background: rgba(0, 0, 0, 0);
    outline: 0;
    color: #FFFFFF
}

QListView::item:selected:hover, QListView::item:selected:hover, QTreeView::item:selected:hover  {
    background: #3d8ec9;
    color: #FFFFFF;
}

QSlider::groove:horizontal {
    border: 1px solid #3A3939;
    height: 8px;
    background: #201F1F;
    margin: 2px 0;
    border-radius: 2px;
}

QSlider::handle:horizontal {
    background: QLinearGradient( x1: 0, y1: 0, x2: 0, y2: 1,
      stop: 0.0 silver, stop: 0.2 #a8a8a8, stop: 1 #727272);
    border: 1px solid #3A3939;
    width: 14px;
    height: 14px;
    margin: -4px 0;
    border-radius: 2px;
}

QSlider::groove:vertical {
    border: 1px solid #3A3939;
    width: 8px;
    background: #201F1F;
    margin: 0 0px;
    border-radius: 2px;
}

QSlider::handle:vertical {
    background: QLinearGradient( x1: 0, y1: 0, x2: 0, y2: 1, stop: 0.0 silver,
    stop: 0.2 #a8a8a8, stop: 1 #727272);
    border: 1px solid #3A3939;
    width: 14px;
    height: 14px;
    margin: 0 -4px;
    border-radius: 2px;
}

QToolButton {
    /*  background-color: transparent; */
    border: 2px transparent #4A4949;
    border-radius: 4px;
    background-color: dimgray;
    margin: 2px;
    padding: 2px;
}

QToolButton[popupMode="1"] { /* only for MenuButtonPopup */
 padding-right: 20px; /* make way for the popup button */
 border: 2px transparent #4A4949;
 border-radius: 4px;
}

QToolButton[popupMode="2"] { /* only for InstantPopup */
 padding-right: 10px; /* make way for the popup button */
 border: 2px transparent #4A4949;
}


QToolButton:hover, QToolButton::menu-button:hover {
    border: 2px solid #78879b;
}

QToolButton:checked, QToolButton:pressed,
    QToolButton::menu-button:pressed {
    background-color: #4A4949;
    border: 2px solid #78879b;
}

/* the subcontrol below is used only in the InstantPopup or DelayedPopup mode */
QToolButton::menu-indicator {
    image: url(images/down_arrow.png);
    top: -7px; left: -2px; /* shift it a bit */
}

/* the subcontrols below are used only in the MenuButtonPopup mode */
QToolButton::menu-button {
    border: 1px transparent #4A4949;
    border-top-right-radius: 6px;
    border-bottom-right-radius: 6px;
    /* 16px width + 4px for border = 20px allocated above */
    width: 16px;
    outline: none;
}

QToolButton::menu-arrow {
    image: url(images/down_arrow.png);
}

QToolButton::menu-arrow:open {
    top: 1px; left: 1px; /* shift it a bit */
    border: 1px solid #3A3939;
}

QPushButton::menu-indicator  {
    subcontrol-origin: padding;
    subcontrol-position: bottom right;
    left: 4px;
}

QTableView
{
    border: 1px solid #444;
    gridline-color: #6c6c6c;
    background-color: #201F1F;
}


QTableView, QHeaderView
{
    border-radius: 0px;
}

QTableView::item:pressed, QListView::item:pressed, QTreeView::item:pressed  {
    background: #78879b;
    color: #FFFFFF;
}

QTableView::item:selected:active, QTreeView::item:selected:active, QListView::item:selected:active  {
    background: #3d8ec9;
    color: #FFFFFF;
}


QHeaderView
{
    border: 1px transparent;
    border-radius: 2px;
    margin: 0px;
    padding: 0px;
}

QHeaderView::section  {
    background-color: #3A3939;
    color: silver;
    padding: 4px;
    border: 1px solid #6c6c6c;
    border-radius: 0px;
    text-align: center;
}

QHeaderView::section::vertical::first, QHeaderView::section::vertical::only-one
{
    border-top: 1px solid #6c6c6c;
}

QHeaderView::section::vertical
{
    border-top: transparent;
}

QHeaderView::section::horizontal::first, QHeaderView::section::horizontal::only-one
{
    border-left: 1px solid #6c6c6c;
}

QHeaderView::section::horizontal
{
    border-left: transparent;
}


QHeaderView::section:checked
 {
    color: white;
    background-color: #5A5959;
 }

 /* style the sort indicator */
QHeaderView::down-arrow {
    image: url(images/down_arrow.png);
}

QHeaderView::up-arrow {
    image: url(images/up_arrow.png);
}


QTableCornerButton::section {
    background-color: #3A3939;
    border: 1px solid #3A3939;
    border-radius: 2px;
}

QToolBox  {
    padding: 3px;
    border: 1px transparent black;
}

QToolBox::tab {
    color: #b1b1b1;
    background-color: #302F2F;
    border: 1px solid #4A4949;
    border-bottom: 1px transparent #302F2F;
    border-top-left-radius: 5px;
    border-top-right-radius: 5px;
}

 QToolBox::tab:selected { /* italicize selected tabs */
    font: italic;
    background-color: #302F2F;
    border-color: #3d8ec9;
 }

QStatusBar::item {
    border: 1px solid #3A3939;
    border-radius: 2px;
 }


QFrame[height="3"], QFrame[width="3"] {
    background-color: #AAA;
}


QSplitter::handle {
    border: 1px dashed #3A3939;
}

QSplitter::handle:hover {
    background-color: #787876;
    border: 1px solid #3A3939;
}

QSplitter::handle:horizontal {
    width: 1px;
}

QSplitter::handle:vertical {
    height: 1px;
}

QListWidget {
    background-color: silver;
    border-radius: 5px;
    margin-left: 5px;
}

QListWidget::item {
    color: black;
}

QMessageBox {
    messagebox-critical-icon    : url(images/critical.png);
    messagebox-information-icon : url(images/information.png);
    messagebox-question-icon    : url(question.png);
    messagebox-warning-icon:    : url(images/warning.png);
}

ColorButton::enabled {
    border-radius: 0px;
    border: 1px solid #444444;
}

ColorButton::disabled {
    border-radius: 0px;
    border: 1px solid #AAAAAA;
}

Solution

Probably the best way to do this is to use QDir.addSearchPath, which would allow you to use a simple alias in your stylesheet like this:

QComboBox::down-arrow
{
    image: url(images:down_arrow_disabled.png);
}

The alias needs to be created at the beginning of the script, like this:

if __name__ == '__main__':
    
    import os
    
    # use script directory as root
    root = os.path.dirname(os.path.abspath(__file__))        
    QDir.addSearchPath('themes', os.path.join(root, 'themes'))
    QDir.addSearchPath('images', os.path.join(root, 'themes/images'))

    app = QApplication(sys.argv)

    file = QFile('themes:DarkBlue.qss')
    file.open(QFile.ReadOnly | QFile.Text)
    app.setStyleSheet(str(file.readAll(), 'utf-8'))
    ...

Note that this mechanism is also used to load the stylesheet itself (although it's not strictly necessary). It's best to avoid using normal relative paths, because they depend on the current working directory which can be changed independently. It's very unlikely to be the same every time your application is run, so you should never rely on it.