diff --git a/frontend/src/components/theme-provider.tsx b/frontend/src/components/theme-provider.tsx
index a582b28..9d9d9d4 100644
--- a/frontend/src/components/theme-provider.tsx
+++ b/frontend/src/components/theme-provider.tsx
@@ -35,11 +35,17 @@ export function ThemeProvider({
   ...props
 }: ThemeProviderProps) {
   const [theme, setTheme] = useState<Theme>(
-    () => (localStorage.getItem(storageKey) as Theme) || defaultTheme,
+    () =>
+      (typeof localStorage !== "undefined"
+        ? (localStorage.getItem(storageKey) as Theme)
+        : null) || defaultTheme,
   )
 
   const getResolvedTheme = useCallback((theme: Theme): "dark" | "light" => {
     if (theme === "system") {
+      // Prerendered HTML has no preference to read; the pre-paint script in
+      // the document head resolves it before React ever hydrates.
+      if (typeof window === "undefined") return "light"
       return window.matchMedia("(prefers-color-scheme: dark)").matches
         ? "dark"
         : "light"