Add support for Window::theme on the web (#2687)

CHANGELOG.md

@@ -11,6 +11,7 @@ And please only add new entries to the top of this list, right below the `# Unre
 - Implement `HasRawDisplayHandle` for `EventLoop`.
 - On macOS, set resize increments only for live resizes.
 - On Wayland, fix rare crash on DPI change
+- Web: Added support for `Window::theme`.
 
 # 0.28.1
 

src/platform_impl/web/window.rs

@@ -409,7 +409,20 @@ impl Window {
 
     #[inline]
     pub fn theme(&self) -> Option<Theme> {
-        None
+        web_sys::window()
+            .and_then(|window| {
+                window
+                    .match_media("(prefers-color-scheme: dark)")
+                    .ok()
+                    .flatten()
+            })
+            .map(|media_query_list| {
+                if media_query_list.matches() {
+                    Theme::Dark
+                } else {
+                    Theme::Light
+                }
+            })
     }
 
     #[inline]

src/window.rs

@@ -1126,7 +1126,7 @@ impl Window {
     /// ## Platform-specific
     ///
     /// - **macOS:** This is an app-wide setting.
-    /// - **iOS / Android / Web / Wayland / x11 / Orbital:** Unsupported.
+    /// - **iOS / Android / Wayland / x11 / Orbital:** Unsupported.
     #[inline]
     pub fn theme(&self) -> Option<Theme> {
         self.window.theme()