Option to set default dashboard background in theming app

[michag86]

Make the default dashboard background configurable. This is important for enterprise users to customize it to their corporate design.

I think the best place for this settings is in settings - design (theming app).

[skjnldsv]

cc @jancborchardt @rullzer @MorrisJobke

[MorrisJobke]

Up to @jancborchardt and @juliushaertl I would say. :)

[juliusknorr]

Implemented by #23588