Add dark color option

Dark color option can be selected under "Grid Settings". Currently it only shows the different themes when in full-screen mode. If you want the dark theme to be everywhere it'll be a lot of extra style to change everything in non-fullscreen mode (all the text and stuff). So that'll take some more time and probably a designer's advice.

The colors are copied from the mockup you sent me, so feel free to change any of them.