Simplify settings theme choice
I removed the storage changes from https://github.com/rust-lang/rust/pull/98765 and only kept the UI changes.
You can test it [here](https://rustdoc.crud.net/imperio/simplify-settings-theme-choice/foo/index.html).
Discussion about this still in progress [on zulip](https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/Last.20part.20of.20settings.20simplification).
r? ````@notriddle````