Yes to more app stores. That's explicitly required in the DMA. Of course it will take time for developers to build their own app stores, but Microsoft, for one, is already hard at work. I suspect the same is true of Epic and others. In reality, stores don't need to be much more than responsive wrappers, so we should see some options fast.

As for themes, app icons, and other customisations, the DMA doesn't touch on that. At least, I didn't see any requirements for that. The legislation is intended to ensure equal competition and access for developers. It's not intended to force Apple to allow greater OS customisation, for example. Unless, of course, Apple gives themselves such abilities in other apps.