I have the same problem... I installed a few new themes and they doesnt work. Only the blackcandy theme works all other themes doesnt work. I tried to change the permissions of the themes folders like the folder blackcandy and also to add ?flush at my URL, but it doesnt works anyway.
i have this problem too..i've tried lots of different themes (even ones without a '_' in them like the previous post said?) and none of them are working..it just comes up with the 'fatal error' etc.
i don't know what i'm doing wrong! because i'm doing everything that it says to!