Could be one of a few things.
Are you in Classic Mode?
Did Windowsblinds "unpatch" your system files but hasn't done a clean job of it?
(I don't know if WB patches system files, never use it). Download
this app. It can patch the three files required to run a 3rd party theme, but more importantly for you, it will tell you if they are patched or not.
(Daft one) Have you rebooted? I would be tempted to do a general clean-up as well as a full Disk Cleanup and reboot.
When you say a blocky Grey Theme, is everything like that or just parts of it? How far down the modified files road are you? if not far it may be worth running
sfc /scannow to put all your system files back to their correct state.
Lastly
(for now) can you put a screen shot up?