[Feature Request] Add a way to use your own background

[flee541]

Possible to add a way to either upload or link to an image to use as the background. Extra CSS options could be if it repeats, stretches, custom sizing etc.

Right now, we need to manually add it via custom CSS which seems like an extra step when it should be part of the theme "or create theme" settings.

[gglue]

It would be cool if they added an option to play a YouTube video as the background as well, although I know that's more of changing the React code rather than CSS