Browse around the
Supercard subforum for questions regarding compatibility, as well as a bunch of feedback from those of us who has the flashcard already.
I've confirmed - for my card, at least - that Emerald, Fire Red, Leaf Green, Sapphire and Ruby do not work on the emulator (white screen that hangs forever).
As for the themes: you can add custom ones, and swap the themes at will via the main menu of the card. I have yet to add any custom ones, but I do think you can use the EOS ones already available. I will try this a little later tonight. For customizations, it's as easy as opening photoshop (or your favourite editor) and editing the BMP files found in an existing theme (saving them as a new theme, of course). For the loading screen, which is hard-coded, you can use
this tool found over at the English Supercard forums, created by Blastar.