How can i set the font size of the tab text for a QTabWidget , with Qt's StyleSheet file ? I just don't know "what" widget is that
QTabWidget { font: 10pt; }
wasn't helping.
It's QTabBar.
Have a look at the Customizing QTabWidget and QTabBar docs for info.
This is how to set its font size:
QTabBar::tab { font: 10pt }