My Fav Resources for Learning CSS
Chris Schwartz-Edmisten
Chris will give you the CSS you need to accomplish your task, but spends the time in his videos to explain it so you can learn as you go.
https://schwartz-edmisten.com/blog/category/Custom+CSS
Becca Harpain from Inside the Square
Becca is awesome because she has CSS code snippets for any customization you can think of. My go to ones are media query
The Squarespace Forum!
If you haven’t found your way to the squarespace forum, do it! They have a database of question and answer threads that are extremely useful. You can also post up question on there as well!