I believe that you only learn when you truly build something. I have tried to learn CSS a lot of time but after learning selectors and some basic properties I forget them because I don't use them a lot.
This time I am thinking to put around 15-20 days continuously but I need some structured course with projects. I am trying to find out but unable to really get what I need. I am aware of freecodecamp.org and I have already finished it in 2 days. All I need is to practice for next 20 days.Please help me with some resources or challenges type project that aims to make you better at CSS.