I would like to add docs about how to use custom TailwindCSS v4 without overriding the theme

some users may have the same question: #682, the solution in https://github.com/bep/hugo-testing-tailwindcss-v4/tree/main may still not be polished and it does not adapt for Hextra.

My current solution:

1. Add a package.json which includes package tailwind/postcss, just like https://github.com/imfing/hextra/blob/main/package.json
2. In assets/css, add styles.css and import original CSS from Hextra:

@import "../../themes/hextra/assets/css/styles.css";

3. When adding your own custom TailwindCSS class, you can compile them using npm run build:css, which is actually running:

npx postcss --config postcss.config.mjs --env production assets/css/styles.css -o assets/css/compiled/main.css

Then done! Your website is rendered with your own custom TailwindCSS class!

Let me know if it is a good idea to add the above to the hextra docs.