Dark scheme for the admin panel

+1. Every one of our moderators has asked for this.

It's on the list for 4.3. Most of the admin panel already uses the shared colour variables; the leftovers are about thirty hard-coded colours.

If it helps, I keep a list of the hard-coded ones from my theme work. I'll post it in a reply here.

Readers get a dark scheme; admins don't. Moderating at night with a white admin panel next to a dark topic list is rough on the eyes.

Proposal: the admin panel follows the same colour scheme the user picked for the site.