Like I love Stripe so what I will do is I'll try to imitate that & I did [0]. For inspiration of websites which have great CSS, check HyperPixel [1]
Take 1 website & try to imitate that. Now don't just go on for 30 days making something that you won't use yourself. Make something useful too. Like a StopWatch like BigTimer [2] or Pomodoro. That way you make a useful application & get great at CSS. Remember its 99% inspiration & 1% perspiration.
TL;DR - Make something that solves a problem & raises your skill level at CSS. Don't make same application or on the same level like you might be making boxes all the time. Don't do that. Raise your level every time. If you are a beginner, do something that requires intermediate level skills. If you are a intermediate, then make something that requires experienced level skills. And while doing that share it with the world. Make it on CodePen so people can learn from you. Small victories will make you happy & you will continue to build something great.
The projects are different, sometimes just HTML & CSS & JS & sometimes just Node JS or something else.
Make your own projects & build it everyday for a year. That way you will learn stuff.
The best course that will teach you Advanced CSS & SASS [1] is from Jonas. It is the absolute best which teaches Cascading & how CSS actually works. If you do that, you won't need anything else other than practice. And if you can't afford it, there are tons of resources on the internet but are scattered everywhere but they will teach you how to do it.
[0]: https://100dayz.js.org
[1]: https://www.udemy.com/advanced-css-and-sass