update README

Author: sakihet

README.md

@@ -48,6 +48,23 @@ Additional features:
 - [vite-plugin-pwa](https://vite-pwa-org.netlify.app/)
 - [wouter](https://github.com/molefrog/wouter)
 
+## CSS
+
+Trellith is designed by Plain Old CSS. It doesn't use any CSS frameworks for minimizing future maintenance costs.
+
+- reset.css
+  - Remove default styles
+- base.css
+  - Add basic styles
+- token.css
+  - Manage design tokens by CSS custom properties
+- utility.css
+  - Utility-First CSS
+- layout.css
+  - Reusable CSS for creating layouts
+- pattern.css
+  - Reusable CSS for creating common UI
+
 ## Contributing
 
 If you find any bugs or have feature requests, please create an issue and let me know.