I'm a bit of a CSS newbie. What little I've learned, I've learned through trial-and-error, and reading the support forums.
One thing I've noticed is that many people ask the same question: how do I change this? Where do I find this? etc.
A major issue for me, and it seems many others, is simply trying to FIND what line of the CSS to change to achieve what I want.
It occurred to me that if the style.css for the default theme was better commented, many people's problems would be instantly solved, and the work of the mods would be halved overnight.
All it takes is a decent explanatory comment above every line/section of the default CSS, such as:
/*This controls the page color: replace the default color with another value of your choice*/
/*This controls the width of the blah-blah-blah*/
/*"section.featured-post" refers to the xxxx xxxxx*/
... etc etc... What this would mean is that all the answers are RIGHT THERE in the very file people are trying to edit (or copy into a child theme), saving bags of time going to the forums and searching/posting, and bugging the mods with the same old questions. It also teaches people what things are called, the correct syntax etc, as they go along.