UI: Don't set theme if it didn't change

If the user just reselects the theme they're already on, we shouldn't
set it again, as setting a theme always introduces issues (that are
especially visible on macOS and Linux).
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f