brand.js: sync shared toggle with legacy body otb-dark class

brand.js

@@ -1,6 +1,13 @@
 (function () {
   function applyTheme(theme) {
     document.documentElement.setAttribute("data-theme", theme);
+
+    if (theme === "dark") {
+      document.body.classList.add("otb-dark");
+    } else {
+      document.body.classList.remove("otb-dark");
+    }
+
     const toggle = document.getElementById("otbThemeToggle");
     if (toggle) toggle.checked = theme === "light";
   }