I get that customization can lead to accessibility issues, unpredictable glitches, and/or vulnerabilities, but I really think that custom theming should be a bigger focus than pushing a major UI redesign on users.
Also the ability as the user viewing another's profile to disable custom theming, as I've seen in Wafrn, for example.
Seriously, I really really like that first thing when viewing a Wafrn profile with custom theming for the first time, it asks you if you want custom theming enabled or not.