Drop theme selector

[oscfdezdz]

Fix #218

[mjakeman]

Thanks for all of these PRs, much appreciated!

It's a busy week for me, so it may take a few days to review them all.

If you have matrix, I'd suggest joining Extension Manager's matrix room. I've created it only recently so there's not much activity, but I'd like to use it to discuss plans/general goals: https://matrix.to/#/%23extension-manager:matrix.org

[mjakeman]

Reviewed, looks great!