Widen support for system theme detection to linux

[TuTiuTe]

Description

Currently, as stated by the documentation getting the system theme is only supported on Windows, web and macOS

I'm interested in linux support, as it's my os and has a quick fix: the crate dark-light, which supports theme detection on a variety of DEs through the use of dbus API.

I'd be happy to submit a PR to add this crate, however I don't know the project's policies on adding dependencies, so let me know if this is something the project wants. Also lmk if it should also replace the current windows / web / macos system theme detection code for homogeneity, as it's multi platform.

Relevant platforms

No response

[kchibisov]

Given that it's dbus based, we don't have a policy yet on adding such things. Though, given new winit design works, there could be a point in the near future with sending winit-like events from 3rd party dependencies.