Comment 38 for bug 1599453

See the attached screenshot. If your systems background colour is black then it will look fine. I'm not sure why mine is set to white, or how/when it got changed, but that is when it becomes a problem as you can see in the screenshot. It only happens for system theme colors as I explained in my previous comment.

I've added the scrollbar track highlight snippet.
Committed revision 1651.

terminator.css is for fixes. Stuff that has to go in to fix rendering problems (Ambiance transparent scrollbars) or functionality (Adwaita oversized hover area).

terminator_styling.css is for optionally modifying looks to improve the appearance for purely aesthetic reasons (Ambiance notebook headers). Some may dislike these kind of tinkerings and prefer an unadulterated default theme, so by keeping it separate we can give the purists the ability to turn of these stylings.

I rebooted overnight, and now strangely the notebook header background is not working?!? Still investigating...