I have the Windows XP theme, slightly modified.

I did modify the colours temporarily, to see how something would look, a few days ago, and then I reset them. I haven't rebooted since then.

You're probably right that modifying the theme is what caused it. I was surprised when I noticed this yesterday, not so much that it was happening, but more that I hadn't noticed it before.

I'll let you know what it looks like after I've rebooted.