Implement ThemeChanged for web target. (#1462)

* Implement ThemeChanged for web target.

* Add TODO upstream to stdweb.

Co-authored-by: Ryan G <ryanisaacg@users.noreply.github.com>

src/platform_impl/web/web_sys/mod.rs

@@ -38,6 +38,17 @@ impl WindowExtWebSys for Window {
     fn canvas(&self) -> HtmlCanvasElement {
         self.window.canvas().raw().clone()
     }
+
+    fn is_dark_mode(&self) -> bool {
+        let window = web_sys::window().expect("Failed to obtain window");
+
+        window
+            .match_media("(prefers-color-scheme: dark)")
+            .ok()
+            .flatten()
+            .map(|media| media.matches())
+            .unwrap_or(false)
+    }
 }
 
 pub fn window_size() -> LogicalSize<f64> {