Add doc

crates/bevy_window/src/event.rs

@@ -293,7 +293,9 @@ pub struct WindowMoved {
     pub position: IVec2,
 }
 
-/// An event that is sent when system window theme has changed.
+/// An event that is sent when system changed window theme.
+///
+/// It is only sent when [`Window::preferred_theme`](crate::window::Window::preferred_theme) is `None`.
 #[derive(Debug, Clone, PartialEq, Eq, Reflect, FromReflect)]
 #[reflect(Debug, PartialEq)]
 #[cfg_attr(

examples/window/window_settings.rs

@@ -18,6 +18,7 @@ fn main() {
                 fit_canvas_to_parent: true,
                 // Tells wasm not to override default event handling, like F5, Ctrl+R etc.
                 prevent_default_event_handling: false,
+                preferred_theme: Some(WindowTheme::Dark),
                 ..default()
             }),
             ..default()