fix:theme-flakiness (#20460)

packages/lib/hooks/useTheme.tsx

@@ -11,7 +11,9 @@ import { useEmbedTheme } from "@calcom/embed-core/embed-iframe";
  */
 // eslint-disable-next-line @typescript-eslint/ban-types
 export default function useTheme(themeToSet: "system" | (string & {}) | undefined | null, getOnly = false) {
-  themeToSet = themeToSet ? themeToSet : "system";
+  if (typeof window !== "undefined") {
+    themeToSet = themeToSet || localStorage.getItem("app-theme") || "system";
+  }
   const { resolvedTheme, setTheme, forcedTheme, theme: activeTheme } = useNextTheme();
   const embedTheme = useEmbedTheme();