egui_demo_app: fall back to dark mode if there is no system preference

egui_demo_app/src/wrap_app.rs

@@ -109,6 +109,12 @@ impl WrapApp {
             }
         }
 
+        if cc.integration_info.prefer_dark_mode == Some(false) {
+            cc.egui_ctx.set_visuals(egui::Visuals::light()); // use light mode if explicitly asked for
+        } else {
+            cc.egui_ctx.set_visuals(egui::Visuals::dark()); // use dark mode if there is no preference, or the preference is dark mode
+        }
+
         slf
     }