Add & enable by default more dark themes + add more hidden light themes

@umop @devoncarew @keertip @gaurave @sraroop

I've been regularly using 3 out of our original 4 dark themes, and haven't noticed any UI problems with them. I've also taken a look at all the themes that Ace supports (dark and light), and at least the dark ones look just as good, if not better, as our only current official choice (Monokai).

This PR adds all of the Ace-supported themes and enabled all the dark ones by default.

It also consolidates config management somewhat.