How to remember all of the css code?

With CSS code the best thing to do (in my opinion) is keep a site like http://www.w3schools.com/css/ in your bookmarks to reference when you need it.

You might reference it a lot at first and then gradually you will remember more and more. (I've done and still do the same with basic HTML as well as CSS)

Hope that helps

Totally agree with Andrew w3schools is a reference for sure. css tricks is less a reference but very useful for specific questions.

keep learning. don't stop and don't hesitate to go back to the basics as much as necessary fo reenforce your knowledge. Try to learn on different web sites.

Also an excellent way is to use codepen.io or jsfiddle to make a lot of html/css tests and bookmarks them. You can also do the same in your drive/dropbox.

Finally a project is a good way to test your knowledge on the field.

An (imo better) alternative to w3schools is the Mozilla Development Network (https://developer.mozilla.org/en-US/).

Also, dont try and remember every single CSS code. The most useful ones will be learned through practicing and experience, and the rest can be found through Google. What's important is knowing what is possible, not how to do it.