Use system color scheme in themes

[RangerMauve]

It'd be useful if we could pull the system color scheme into Agregore's color scheme.

Then we could have a flag to enable this functionality if you want apps to match the system somewhat. (when importing agregore://theme/style.css)

Electron has an API for this in MacOS and Windows https://www.electronjs.org/docs/latest/api/system-preferences#systempreferencesgetcolorcolor-windows-macos

However this will need to be expanded with some way of getting the system theme in Linux from GNOME and from Plasma Mobile. (not sure if we should somehow get X themes in here too?)

This will be useful for Agregore Mobile as well if we can get the system color scheme from the Android preferences.

The goal here is to make it easier to style p2p web apps to be cohesive with the rest of the host OS.

[RangerMauve]

I think we'll also want to take window transparency into account to support window managers with that fancy blur effect: https://www.electronjs.org/docs/latest/tutorial/window-customization#create-transparent-windows