Comment 6 for bug 195929

I agree, I was wrong to turn off HTML themes before, we just have to ship them and wait for GTK theme authors to fall into line.

Any attention you can draw to this issue would be helpful :-).

The basic issue here is that theme authors assume that widgets are being drawn on top of the theme's designated window background color (in the case of your screenshot, white). So they often draw borders and bevels using that background color around the outside of the widget. This gives the results you see.

Basically theme authors need to stop making assumptions about the window background color and try to make their themes look good no matter what that background color is. This is hard in some cases but in other cases it's simply a matter of using transparency instead of drawing the window background color as part of the widget.