Adam Engst, TidBITS:

Some months ago, I noticed that something was slightly wrong when I was using Google Chrome. I’d type something in the address bar and get the auto-complete suggestions appearing below, but when I arrowed down to select one of the items in the list, I couldn’t tell which was selected. It was as though I’d lost the gray highlight color.

And:

I normally have the background of BBEdit documents set to a very pale yellow, but they had somehow reverted to white. Plus, Web pages with light backgrounds were also showing with a garish, glaring white. Again, restarting and standard troubleshooting made no difference.

Follow the link for the whole detective story, and a lesson learned about Accessibility settings and keyboard shortcuts.