hi Claire,
I have krystle running on my 2.5 development server in work - in fact I've based our new theme for when we upgrade to 2.5 this summer on it - so yes at the moment it works fine on 2.5 and I'm not aware of anything that will prevent it doing so (or any other 2.4 themes).
That said, 2.5 is not finalised yet and there may be some things that will need to be tweaked in krsytle to get the best out of 2.5 before 2.5 is released, but those should be relatively minor (I hope!) :D
If you want to make any other customisations, it may be better to start a new thread, just to make it clear to other readers what you are trying to solve at the time
Richard