Rayn19: it is normal behaviour - custom theme fails to load when you don't bypass theme signature verification. nothing to do. Also other solution such as uxtheme patch or uxstyle has completely same problem - when you apply custom theme and then disable the software (or just windows update break something), DWM will fail.
On the other side, no need to refresh the OS. It is enough to boot into recovery console, run regedit, connect your registry and change the theme manually back.
On Windows Vista, it would revert to Windows Classic.
On Windows 7, it would revert to the Windows 7 Aero theme.
I thought that Windows 8 would have this go to the Windows 8 "basic" visual style (the same kind used in the high contrast themes).
The reason I say this is because when I had the Windows 8 Enterprise trial in a VM, I did something really sneaky and deleted both the aero.msstyles and aerolite.msstyles, and what Windows 8 did was recreate them in their respective folders (I deleted them in the recovery command prompt, when they weren't loaded) and loaded the basic theme.
Why won't Windows 8 do that here? You would think that MS would have wanted more resiliency than that in their OS. So someone just needs to modify the aero.mssyles file, and the computer is broken?
And I also wonder why exactly Microsoft is so hell-bent on making Windows restricted to just their themes. Why not just open up msstyles development as a selling point for the OS? After all, it is a personal computer, right?