Add `pygame.system.get_theme()` (w/ sdl3)

[damusss]

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I implemented fully what the title says, "fixing" half of https://github.com/pygame-community/pygame-ce/issues/2760#issuecomment-2496022434

The codebase doesn't compile with sdl3 yet, so I'm not sure what kind of versionadded to put.

Depending on which PR gets in before compared to https://github.com/pygame-community/pygame-ce/pull/3296 the other will have to be updated I guess.