docs(theme): add note about color definition (#3976)

Co-authored-by: Benjamin Canac <canacb1@gmail.com>

docs/content/1.getting-started/3.theme.md

@@ -229,6 +229,10 @@ export default defineConfig({
 
 ::
 
+::caution
+When configuring your theme colors, you must use either color names from the [default Tailwind palette](https://tailwindcss.com/docs/colors) (like 'blue', 'green', etc.) or reference custom colors that you've previously defined in your [CSS file](#theme).
+::
+
 ### Extend colors
 
 ::framework-only