eframe: Figure out dark mode preference on native

[emilk]

egui_web follows the user preference for light vs dark mode, and it would be nice if we could do the same when compiling natively (somewhere in egui-winit).

EDIT: as @parasyte points out, this should be fixed in winit: rust-windowing/winit#1549

[parasyte]

This would be nice to have! AFAIK, winit only has support for probing the system theme on Windows, X11, Wayland, and web.

One of the relevant winit issues: rust-windowing/winit#1549

[BrainBlasted]

For Linux, you may want to look into the cross-desktop color-scheme key provided by xdg-desktop-portal: flatpak/xdg-desktop-portal#633

[LordMZTE]

Wouldn't it be great, if eframe would actually not just figure out dark/light mode on linux, but actually used the GTK3 theme? If we're adding platform theme support, then that should be easy to do.

[Be-ing]

There is a dark-light crate that does this.

For Linux, you may want to look into the cross-desktop color-scheme key provided by xdg-desktop-portal: flatpak/xdg-desktop-portal#633

That is being implemented in the dark-light crate: frewsxcv/rust-dark-light#4

[emilk]

Oooh nice find @Be-ing! Let's start using it right away so we don't need to wait for winit.