Well, if you haven't turned it off, it will most likely still be on.
Start > Help and Support
Under "Pick a Task", you should see an option labeled "Undo changes to your computer with system restore". Click it, and if system restore is enabled, you should get a calender brought up with several "restore points" listed on it. Choose the one closest to the point before you switched your theme, and cross your fingers that it works.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w