Night mode CSS

Further cover for spotlight/promo links
Misc CSS tweaks
Big editor button image

Firefox doesn't like separate x/y values for background-position
Big editor button layout

Margin was being ignored as it wasn't specific enough. Force box-sizing
on the textarea so that the button sits flush with the right side
instead of a few px off.
Merge pull request #394 from gavin19/master

CSS misc/night mode