Where can I find a complete list of all possible zenPhoto CSS Tags?

There is not such a list, the closest would be one of the theme's css files as you might have guessed.

Generally you can always assign your own css classes and ids (most functions that print html have a $class and/or $id parameter). Also for nearly all functions a get version that only returns the pure values exists so that you can make your own html output (if you are able to of course knowledgewise).

Generally you have the nearly absolute freedom to use whatever you want in your theme. You don't need to use the setup and naming the standard themes use.

ok,.... so if I take the "default" theme as an example I have almost everything basic, I think.

Yes, that should feature all general tags.