fix: undefined document in ssr

gatsby-ssr.js

@@ -14,6 +14,8 @@ const ThemeScript = () => {
     else return "dark"
   }
 
+  if (typeof document === "undefined") return;
+
   document.documentElement.style.setProperty(
     "--initial-theme",
     getInitialTheme()

src/ThemeProvider.tsx

@@ -26,9 +26,11 @@ const ThemeProvider: FC<IScopedDownChildren> = ({ children }) => {
 
   // App's current theme
   const [theme, setTheme] = useState<ThemeType>(
-    document.documentElement.style.getPropertyValue(
-      "--initial-theme"
-    ) as ThemeType
+    typeof document !== "undefined"
+      ? (document.documentElement.style.getPropertyValue(
+          "--initial-theme"
+        ) as ThemeType)
+      : "dark"
   )
 
   const themeObject = useMemo(