Optional resize-background theme_setup variable

[JartanFTW]

Summary

Upon passing in the optional variable resize_background from theme_setup.json, Horizon would re-size the background to match the input.

Motivation

This would allow people to simple replace existing theme backgrounds with new ones, without potentially breaking the theme by having later images/text pasted on in a way that overflows past the borders of the background.

Additional Information

Must be optional as to prevent outdated theme incompatiblity.

[JartanFTW]

On-top of this, can also add background transparency optional argument, which would work the same way on surface-level.