feat: follow system theme

just sync it with the os 🥴

[AnonymousWP]

This should be an option; not the default.

[Rudxain]

I'm willing to contribute to this. However, I'm still trying to get familiarized with the codebase.

dark-light doesn't provide async (It hasn't stabilized, yet) API, so color-scheme can only be efficiently synced on app startup. We could poll dark_light::detect to update in real-time, but that seems overkill since most users won't frequently change the DE theme while UAD is running. Maybe we could update on other events such as window-focus?

In the future, we could copy some code from Firefox (is MPL-2.0 compatible with GPL-3.0?), to update in real-time in an event-driven way

[AnonymousWP]

It's fine if it's only on start-up. If it's not too much work it's a nice bonus, but not mandatory for sure.🙂

[Rudxain]

#650 #627