Lots of CSS and HTML help from these sites:

https://www.w3schools.com/
https://www.geeksforgeeks.org/
https://css-tricks.com/

Most notably:
https://css-tricks.com/snippets/css/a-guide-to-flexbox/
https://css-tricks.com/snippets/css/complete-guide-grid/

Throughout my css files, I have commented where I have heavily referenced css code.