Hi @RachelSalmon,
The following css may help a bit:
.canvas-rce__skins--root.rce-wrapper span span button span span {
color:#000000;
}
Note: you can set the color to whatever you'd like, I just did black to ensure I could see that this worked.
I tried this is a few places (announcements, assignments, pages) and it did seem to work in all of those areas. I would not expect it to work in New Quizzes though, since that's technically an external tool where normal Canvas CSS doesn't apply. I'm also not 100% sure if there will be any other elements unexpectedly altered by this CSS or not. Someone else in the Developers Group may have a better suggestion, so I'll move this thread over there.
Long-term, I think this needs to be an Instructure fix to theming though, as we should not have to resort to CSS (which can break with future Canvas updates) for what I feel is a core functionality need. This is just my opinion...
Hope this helps a bit.
-Chris