Add ActiveEventLoop::system_theme()

This also fixes macOS returning `None` in `Window::theme()` if no theme
override is set, instead it now returns the system theme.

MacOS and Wayland were the only ones working correctly according to the
documentation, which was an oversight. The documentation was "fixed"
now.

Fixes #3837.

src/platform_impl/web/event_loop/window_target.rs

@@ -614,6 +614,16 @@ impl ActiveEventLoop {
         self.runner.listen_device_events(allowed)
     }
 
+    pub fn system_theme(&self) -> Option<Theme> {
+        backend::is_dark_mode(self.runner.window()).map(|is_dark_mode| {
+            if is_dark_mode {
+                Theme::Dark
+            } else {
+                Theme::Light
+            }
+        })
+    }
+
     pub(crate) fn set_control_flow(&self, control_flow: ControlFlow) {
         self.runner.set_control_flow(control_flow)
     }