Follow the System Theme in egui (#4860)

* Some initial progress towards #4490

This PR just moves `Theme` and the "follow system theme" settings to
egui and adds `RawInput.system_theme`.
A follow-up PR can then introduce the two separate `dark_mode_style` and
`light_mode_style` fields on `Options`.


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### Breaking changes

The options `follow_system_theme` and `default_theme` has been moved
from `eframe` into `egui::Options`, settable with `ctx.options_mut`

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui-winit/src/lib.rs

@@ -14,7 +14,7 @@ pub use accesskit_winit;
 pub use egui;
 #[cfg(feature = "accesskit")]
 use egui::accesskit;
-use egui::{Pos2, Rect, Vec2, ViewportBuilder, ViewportCommand, ViewportId, ViewportInfo};
+use egui::{Pos2, Rect, Theme, Vec2, ViewportBuilder, ViewportCommand, ViewportId, ViewportInfo};
 pub use winit;
 
 pub mod clipboard;
@@ -111,6 +111,7 @@ impl State {
         viewport_id: ViewportId,
         display_target: &dyn HasDisplayHandle,
         native_pixels_per_point: Option<f32>,
+        theme: Option<winit::window::Theme>,
         max_texture_side: Option<usize>,
     ) -> Self {
         crate::profile_function!();
@@ -150,6 +151,7 @@ impl State {
             .entry(ViewportId::ROOT)
             .or_default()
             .native_pixels_per_point = native_pixels_per_point;
+        slf.egui_input.system_theme = theme.map(to_egui_theme);
 
         if let Some(max_texture_side) = max_texture_side {
             slf.set_max_texture_side(max_texture_side);
@@ -403,6 +405,13 @@ impl State {
                     consumed: false,
                 }
             }
+            WindowEvent::ThemeChanged(winit_theme) => {
+                self.egui_input.system_theme = Some(to_egui_theme(*winit_theme));
+                EventResponse {
+                    repaint: true,
+                    consumed: false,
+                }
+            }
             WindowEvent::HoveredFile(path) => {
                 self.egui_input.hovered_files.push(egui::HoveredFile {
                     path: Some(path.clone()),
@@ -462,7 +471,6 @@ impl State {
             | WindowEvent::Occluded(_)
             | WindowEvent::Resized(_)
             | WindowEvent::Moved(_)
-            | WindowEvent::ThemeChanged(_)
             | WindowEvent::TouchpadPressure { .. }
             | WindowEvent::CloseRequested => EventResponse {
                 repaint: true,
@@ -890,6 +898,13 @@ impl State {
     }
 }
 
+fn to_egui_theme(theme: winit::window::Theme) -> Theme {
+    match theme {
+        winit::window::Theme::Dark => Theme::Dark,
+        winit::window::Theme::Light => Theme::Light,
+    }
+}
+
 pub fn inner_rect_in_points(window: &Window, pixels_per_point: f32) -> Option<Rect> {
     let inner_pos_px = window.inner_position().ok()?;
     let inner_pos_px = egui::pos2(inner_pos_px.x as f32, inner_pos_px.y as f32);