Just as I was browsing the site, I saw the vote and favorite buttons lose both all their color and all their brightness:
I also see that the "Similar Questions" page has lost all its color:
Additionally, the "[x] question(s) with new activity" bar has lost its background color:
This seems to have happened on all sites. I can reproduce this in Microsoft Edge and in Internet Explorer 11, but not in Google Chrome Canary. Regardless, can this please be fixed?
Update: It seems to have been fixed on Meta Stack Exchange, but it still exists on other sites in Edge, IE11, and (per Pierre.Vriens's answer) iOS Safari.