Custom light & dark themes on system theme change

[kernelkind]

Is your feature request related to a problem? Please describe.
Currently, when the system theme changes, the Visuals theme is switched to the hard-coded egui::Visuals::dark() or egui::Visuals::light(). I would like for there to be an option to use custom Visuals for dark and light themes.

This seems to be where the system theme change is detected and the egui visuals are set:

egui/crates/eframe/src/native/epi_integration.rs

Line 271 in c3f386a

self.egui_ctx.set_visuals(theme.egui_visuals());

And this seems to be where the hard-coded Visuals are set:

egui/crates/eframe/src/epi.rs

Lines 501 to 506 in c3f386a

	pub fn egui_visuals(self) -> egui::Visuals {
	match self {
	Self::Dark => egui::Visuals::dark(),
	Self::Light => egui::Visuals::light(),
	}
	}

Describe the solution you'd like
There should be some way to pass a Visuals that corresponds to the dark theme, and another Visuals which corresponds to the light theme. Then, when a theme change is needed, those custom themes would be used instead of the hard-coded egui::Visuals::dark() or egui::Visuals::light().

Describe alternatives you've considered
None

Additional context
I am working on https://github.com/damus-io/notedeck and we would like to support custom theming at some point. It's not a high priority at the moment, and I would be open to submitting a PR for this feature in the future.

[emilk]

100% agree with this.

The global style is stored in Options::style, and I suggest we split that into three fields:

struct Options {
    pub dark_mode_style: Arc<Style>,
    pub light_mode_style: Arc<Style>,
    pub theme: Theme, // Dark or Light
    …
}

with enum Theme { Dark, Light } (move the one in eframe into egui).

PRs welcome!