Custom gradient themes (low-priority)

[novacrazy]

Add some basic fully-custom theming options using two colors and an angle for the gradient. On the backend these can just be shoved into the prefs JSON structure easily enough.

This would override the temperature-based theme, and set all custom-theme colors to variants of rgba(0, 0, 0, X) where X is some alpha value to give density/layering to the UI.

Look into color-contrast() to determine how to best set the text color.