eframe: Automatically change theme when system dark/light mode changes (#2750)

* React to ThemeChanged event from winit

* React to theme change using media query change event in WASM

* Share conversion from bool -> Theme

* Suppress too_many_arguments warning

* Document limitations of automatically following the dark vs light mode preference

* Simplify expression

* Conditionally compile code to prevent unused item warnings

* Remove needless borrow

* Remove another needless borrow

* Make associated functions to standalone

* Request repaint after theme has changed

* Only install event listener when `follow_system_theme` is enabled

* Remove dark-light feature gate

* Detect system theme using winit

* Update documentation

* Fix typos

* fix warning about unused argument

---------

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

crates/eframe/src/web/events.rs

@@ -207,6 +207,27 @@ pub fn install_document_events(runner_container: &mut AppRunnerContainer) -> Res
     Ok(())
 }
 
+pub fn install_color_scheme_change_event(
+    runner_container: &mut AppRunnerContainer,
+) -> Result<(), JsValue> {
+    let window = web_sys::window().unwrap();
+
+    if let Some(media_query_list) = prefers_color_scheme_dark(&window)? {
+        runner_container.add_event_listener::<web_sys::MediaQueryListEvent>(
+            &media_query_list,
+            "change",
+            |event, mut runner_lock| {
+                let theme = theme_from_dark_mode(event.matches());
+                runner_lock.frame.info.system_theme = Some(theme);
+                runner_lock.egui_ctx().set_visuals(theme.egui_visuals());
+                runner_lock.needs_repaint.repaint_asap();
+            },
+        )?;
+    }
+
+    Ok(())
+}
+
 pub fn install_canvas_events(runner_container: &mut AppRunnerContainer) -> Result<(), JsValue> {
     let canvas = canvas_element(runner_container.runner.lock().canvas_id()).unwrap();