How to hide blue color bottom border of selected file, as shown in the below image, I am using Spring Tool Suite 4.
There is no UI to change this, you will have to edit the Eclipse CSS.
The CSS is in the "plugins/org.eclipse.ui.themes_xxxx/css" folder in your Eclipse installation (xxxx varies depending on your Eclipse release).
For the dark theme I think this is probably the CTabFolder.active
selector in the e4-dark_tabstyle.css
file:
CTabFolder.active {
swt-selected-tab-highlight: #316c9b;
swt-selected-highlight-top: false;
}