fix doc: copy `app-css.md` from `src/app.css`, add `--tw-empty: '';` in `html` block

<!-- Thanks for creating this pull request 🤗 Please make sure that the pull request is limited to one type (docs, feature, etc.) and keep it as small as possible. You can open multiple prs instead of opening a huge one. --> <!-- If this pull request closes an issue, please mention the issue number below --> ## 📑 Description <!-- Add a brief description of the pr --> fix errors in doc `app-css.md`. I found app.css and app-css.md basically the same, while app-css.md has errors, so I copy `app-css.md` from `src/app.css`, add `--tw-empty: '';` in `html` block. <!-- You can also choose to add a list of changes and if they have been completed or not by using the markdown to-do list syntax --> ## ✅ Checks <!-- Make sure your pr passes the tests and do check the following fields as needed - --> - [x] My pull request adheres to the code style of this project - [x] My code requires changes to the documentation - [ ] I have updated the documentation as required - [x] All the tests have passed ## ℹ Additional Information <!-- Any additional information like breaking changes, dependencies added, screenshots, comparisons between new and old behavior, etc. -->