Here is a simple WordPress plugin for applying a dark color-scheme to the WordPress editing pane:
It’s a little raw, and I didn’t mess with the TinyMCE editor, so it doesn’t really make sense for “Visual” mode.
I keep this repository for all my development environment config files. Whenever I start up a new Linux machine, the first thing I do is pull these files, and install them in their respective directories. This way, I can get up and running comfortably and quickly. Feel free to borrow whatever you like.