Add support for custom theme backgrounds (images, colors)

[ghost]

The FuffyChat client has this option, however FluffyChat is not mature or stable enough to be a perfect solution.

I would like this feature added, unless it is too hard to implement.

Describe alternatives you've considered If images would be too hard to get working, having the option for a custom color is also good.

Additional context

[SpiritCroc]

Probably not too hard to get working, but possibly somewhat time-consuming for a feature I don't need myself, so... maybe

[SpiritCroc]

Related upstream issue: https://github.com/vector-im/element-android/issues/2425