This document has been floating around for a number of years now (see General developer forum: Extending Coding Styles) and we've known the need for CSS guidelines to help us move towards building more consistent implementation of styles across the project.
Please can I request you review this draft document and start giving your feedback here on what is contained within it? We are particularly interested to hear things that you think are completely unworkable with Moodle styling or guidelines which you think are missing.
Once we've agreed these guidelines we will then attempt to start enforcing the style on the implementation of new features into Moodle core.
While these rules won't affect what you do in your own themes (unless you choose to observe them), your experience creating themes and how these guidelines might affect them is immensly valuable.
Finally, implementing these guidelines is just one of a few efforts to improve styling/consistency in Moodle core (the Element library is another) - please can I request that in this discussion we stick to discussing this guidelines document only.