Color input field for dark variant

Taken from https://github.com/Codeux/Textual/pull/407 by @foldericon.
Make dark variant use Solarized colors

This should be considered a work in progress I suppose but it's looking good
enough to stop tweaking for now. What would be really nice is to be able to
move all of these colors into the themes and give them names that makes it
possible to easily figure out when and where each is used. That's a big hack
to make on somebody else's product though so I think I'm going to pass for
now.