Welcome to the Vista Forums.
I'm not sure I understand what happened here. You tried to install a Mac theme and now you can't boot into Vista??? Most OEM comps have a recovery partition. If you fill out your system specs (under 'User CP') we may be able to help you further.
If you have the owners manual for the lappy, the info on how to get to the recovery/repair features will be in there.
You could also try 'safe mode'. Info here if needed: Safe Mode
Hope you enjoy the forums.
Please keep us posted.