Do themes really affect how Mullvad Browser operates? I think I’ve read that some things can be changed if they don’t affect the fingerprint directly. For example, editing user.js or installing additional extensions is discouraged, but changing your homepage, bookmark toolbar settings, or default search engine is okay. I believe you can also disable or uninstall the Mullvad Browser Extension that comes with the browser without any worries.
Possibly relevant: About changing settings in Mullvad Browser - #5 by ruihildt