Option to change background color

[nmeo]

When images don't fit the aspect ratio there will be bars on either side that currently is always a specific shade of grey. It would be nice to have the option to change this color to something like black or another color that would fit better with the slideshow on certain tablets.

[ScanuNicco]

I've been looking into the implementation of this and it should be quite simple actually. This library adds a color picker Preference which would make this easy to add to the settings menu, and the rest is really simple.