As you can see here, I have completed the second tile by accepting an answer to one of my questions. However, there is no indication of this accomplishment unless I physically mouse-over the tile and look at the text to the right:

I'm guessing this is a bug since one would expect the tiles to turn blue or something when completed. If this is not a bug, then I humbly propose it as a feature-request.

Edit: this must be a bug. The actual sprite used in the CSS contains two rows of images - one light grey (as pictured above) and one blue.