docs: update Tailwind v4+ guidelines with configuration guidelines

Signed-off-by: Pushpak Chhajed <[email protected]>

Author: pushpak1300

.ai/tailwindcss/4/core.blade.php

@@ -1,7 +1,14 @@
 ## Tailwind 4
 
-- Always use Tailwind CSS v4 - do not use the deprecated utilities.
+- Always use Tailwind CSS v4.1+ - do not use the deprecated utilities.
 - `corePlugins` is not supported in Tailwind v4.
+- In Tailwind v4, configuration is CSS-first using the `@theme` directive — no separate `tailwind.config.js` file needed.
+@verbatim<code-snippet name="Extending Theme in CSS" lang="css">
+@theme {
+  --color-brand: oklch(0.72 0.11 178);
+}
+</code-snippet>
+@endverbatim
 - In Tailwind v4, you import Tailwind using a regular CSS `@import` statement, not using the `@tailwind` directives used in v3:
 @verbatim
 <code-snippet name="Tailwind v4 Import Tailwind Diff" lang="diff">