Okay, played with it a bit more, While trying to replicate what I know of your configuration. For the tests, I'm running a copy of Windows 7 Basic in a Virtual PC so no Aero. But that shouldn't be an issue.
So here's the a default T-Clock with a theme applied:
And, here's what happened when I switched to Classic (You should recognize this one):
Then I switched the text color to black and got this:
Then switched back to white for this:
Then I fiddled with it for a bit trying to figure out how to best reproduce number 2.
There does seem to be a problem with one of the shades of white when entering classic mode...But hitting Refresh T-Clock did clear it up. Black seemed to do just fine at all points. I could not however manage to get it to do the whited out background trick with a theme applied.
So I'm back to pondering your configuration to see if there is anything else I may need to replicate to reproduce the behavior.