After I accidently created a new theme, I realised there was no way to actually delete it. What the hell?
Wheres the frikken delete option!?
Well, luckily theres always the good old long way.
Paste the following into the Start menu (or run dialog):
explorer "%USERPROFILE%\AppData\Local\Microsoft\Windows\Themes"
Search for the corresponding theme file, delete it then proceed to sip on a pinacolada.
Note: It'd be a good idea to keep the custom one.