Allow uploading custom background images

Currently it seems that in order to set a custom background image you have to create a whole new theme. It'd be nice to instead have the ability to upload a custom background image to use with the current (or indeed any) theme. I'm imagining a setting for the background image with the following options:

• Theme default (whatever background the theme specifies)
• Colour (solid or gradient (like button colour options))
• Upload image