Where I can practice CSS??
The best practice is work in own projects.
Go to Learn section, click on the course name in the top, click Discover New Courses, select CSS
Which website is best to practice CSS??
In the playground. Select web.
Make projects, try to recreate design of any website, make UI clone. This is amazing practice, you will learn how to make real websites, and also you can inspect code and see how site is build and styled.