dark_theme: Add automatic color scheme support for spectators.

Similar to the light/dark theme support for the spectators, this adds
the automatic color scheme support by storing the user's preference
in the local storage.

web/src/ui_init.js

@@ -429,6 +429,8 @@ export function initialize_everything(state_data) {
             dark_theme.enable();
         } else if (preferred_theme === "light") {
             dark_theme.disable();
+        } else {
+            dark_theme.default_preference_checker();
         }
     }