[Feature] Dark theme for toolbelt (sidebar on the right)

Required information for bugs. I probably can't help you unless you include this:

• iTerm2 version: Build 2.9.20160206

Dark themed tabs + Solarized dark. Next to that the Toolbelt burns one's eyes being all white :)

Milestone changed to Future Release

I added a similar issue here: https://gitlab.com/gnachman/iterm2/issues/4237

It is a stark contrast having that white toolbelt.

Yeah these two can be merged then I guess.

Dup of #4237

Status changed to closed