Very resilient to browser updates & other add-ons. Somehow, themes on Firefox/Chrome always would cause a bug every few months and I ended up just removing them.
Zero, or next to zero, performance degradation. Hopefully, this is a given, but themes should have very limited privileges and abilities so they don't impact the overall browser speed/responsiveness. Changing colors here or there shouldn't cause lower performance, so keeping a tight leash is key.
Yes! And, the resiliency, hopefully, too: browsers like Chrome get updated so often that I do worry themes will kill this feature or that feature. :( Hopefully, Microsoft Edge themes can restrict that somewhat.
The initial uptake of themers might be small, but hopefully the transition will be easier as there will be "less" to theme.