If you want to test a theme out in an existing course or site but don’t want to change the experience for all users while you test features, User Themes is a great way to change a theme across a site (where themes aren’t forced at the course level) so that you can bang on a theme to find any issues it might introduce for users.
To enable user themes, go to Site Admin > Appearance > Themes > Theme Settings
Once enabled you (and any other user) can edit their theme when editing their profile (note that you can restrict what themes are available by name on the same page above).
Pretty handy. What else can you do with Theme Settings, check it all out here: https://docs.moodle.org/27/en/Theme_settings
Hat tip Richard Oelmann who suggested this in a forum when I was searching for a similar answer.