There'll be a Theme maker application released soon that'll make it very easy to create themes for Maemo. Anyway with the OS being open source there's no limit to customisation.
In LCARS there isn't a thing that wasn't themed. There is a new startup screen, icons, buttons, colour palette, sounds, wallpaper... everything you can imagine. And installing is just a one-click thing.
Take a look: https://synthesize.us/Maemo_themes/LCARS_complete

I don't know how the integration with the OVI store will be made, because that's not announced yet. I assume it'll work like with any other phone.