Add background color in the GUI

A color palette is now available at the bottom of the 'Options' tab;
default color is now black. The palette offers black and white as
options but about every color with the manual color picker.

This fiexes #9

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Note that if merging this before #12, there might be some broken tests in #12 as I'm modifying the footer. I'm putting this here to get feedback though it should be on it's final form IMHO. By all means **merge #12 before this pull request**, I rather solve issues here than in #12.