use colorpicker instead of themes

[Huhni]

This is a pretty simple fix for #6, the colorpicker could be made prettier instead of using the default one for input type color and maybe theme should be renamed now, but it works.

Unfortunately the white flashing issue #3 can only be solved with a fixed color in the app.css or as inline style. Loading the custom color from options is too slow, the best I could do was to turn the bright white flash into a less noticable black flash by adding background-color: #000 to both body and iframe.

resolves #6

[MethodGrab]

I've fixed this with #9 instead because I wanted to keep the 'none' option.

Keeping the 'none' option is important otherwise sites that do not specify a background colour will end up with the custom background colour from this extension when displayed in the iframe.

I'm still looking into ways to resolve #3 without affecting the background colour of pages inside the iframe that do not specify a background colour.