gear_menu: Allow user to configure preferred theme.

We save the preferred theme in localstorage so that user doesn't
have to re-select the theme on every reload. Users on slow
computers might see flash of a theme change, if it happens.

static/js/dark_theme.js

@@ -0,0 +1,26 @@
+import $ from "jquery";
+
+import {localstorage} from "./localstorage";
+import {page_params} from "./page_params";
+
+export function enable() {
+    $("body").removeClass("color-scheme-automatic").addClass("dark-theme");
+
+    if (page_params.is_spectator) {
+        const ls = localstorage();
+        ls.set("spectator-theme-preference", "dark");
+    }
+}
+
+export function disable() {
+    $("body").removeClass("color-scheme-automatic").removeClass("dark-theme");
+
+    if (page_params.is_spectator) {
+        const ls = localstorage();
+        ls.set("spectator-theme-preference", "light");
+    }
+}
+
+export function default_preference_checker() {
+    $("body").removeClass("dark-theme").addClass("color-scheme-automatic");
+}