From what I know, GNOME doesn’t officially support themes and the developers are very adamant that you don’t use them. If you really want to customize your desktop, then you’ll probably be better off with KDE. Do note that while you can rebase your current installation it’s recommended that you completely reinstall the operating system.

They have ISOs with Nvidia stuff pre-installed. I haven’t used it, though.

I found a guide on setting up Bazzite for modded Skyrim. I don’t know how accurate or up to date it is because I don’t have Skyrim, but it’s probably a good place to start.

Another option is Nobara. It’s Fedora-based and gaming focused, while not being immutable. Additionally it’s developed by Glorious Eggroll himself.