Replace usage of theme() to var() as recommended by Tailwind v4 (#1992)

At the [Using the theme()
function](https://tailwindcss.com/docs/upgrade-guide#using-the-theme-function)
section docs advice to use `var()` over `theme()`, but there is a part
of code where `theme()` is still used

Author: Blaumaus

src/docs/upgrade-guide.mdx

@@ -413,7 +413,7 @@ You probably won't even notice this change (it might even make your project look
 @layer base {
   input::placeholder,
   textarea::placeholder {
-    color: theme(--color-gray-400);
+    color: var(--color-gray-400);
   }
 }
 ```