The solution is an undocumented trick:

QTabBar { background-color: transparent; qproperty-drawBase:0; }

This lets you make the dark grey area (called the "tab bar base") as transparent. Or, alternatively, you can style it by changing the background-color value to a color.

@shengfa.zhang
try to call on tabBar().adjustSize() each time, when drugging is over...
I had the same problem...

Great !

Thanks for sharing your solution :)

@Pranjal-Jain
Not sure how one would do that from code directly but you can use stylesheet
setStyleSheet("QTabBar::tab { height: 100px; width: 100px; }");
and control it that way.

Thanks mrjj and Chris Kawa for your valuable response.
Actually I want to create my own customized scroll-button for tabBar with either end button( two buttons like << and >> including qt already provided scroll-buttons < and >) So that we can directly go to the fist or last tab. In Qt scroll-button for Qtabbar provides only step forward or backward functionality means we can scroll only one tab at a time.
Suppose my QTabBar contains 100 tab and currently visible tabs are 55,56,and 57 and if we want to go on first then we have to click approx 49 times or stay in clicked state for few seconds.

if you guys have any approach for doing this efficiently then please. It would be highly appreciated.

What if you invert the styling ? Doing the QTabBar before the rest