It’s not just about the background color of scrollbar!
I’m working on changing gtk-2.0 scrollbar to something like overlay scrollbar these days, and found that this version of Thunderbird (with round corners of tabs) does not follow the gtk-2.0 color scheme for ACTIVE mode too (ACTIVE mode is when you hold the mouse button on the slider to slide it).
It’s not just about the background color of scrollbar!
I’m working on changing gtk-2.0 scrollbar to something like overlay scrollbar these days, and found that this version of Thunderbird (with round corners of tabs) does not follow the gtk-2.0 color scheme for ACTIVE mode too (ACTIVE mode is when you hold the mouse button on the slider to slide it).