CSS specifications are maintained by the [World Wide Web Consortium (W3C)](https://www.w3.org/).
You can build some pretty amazing things in CSS alone, such as this pure-CSS [Minesweeper game](https://codepen.io/bali_balo/pen/BLJONk) (which uses no JavaScript).
A good start is the freeCodeCamp curriculum [Introduction to Basic CSS](https://learn.freecodecamp.org/responsive-web-design/basic-css).
Another suggestion for beginners is W3C's [Starting with HTML + CSS](https://www.w3.org/Style/Examples/011/firstcss) teaches how to create a style sheet.
The site [CSS Zen Garden](http://www.csszengarden.com/) is a great example of how the same HTML code/structure can be styled to look different in unique ways.