It's been pointed out (by poltawski in the comments of MDLSITE-1774) that Moodle doesn't actually have an official CSS style guide, though many people think it does.

There were some suggestions gathered here a couple of years ago:

http://docs.moodle.org/dev/CSS_coding_style

But it's never made it through any official approval process.

And there's some discussions started recently in forums when I created an issue to apply the suggestions above:

https://tracker.moodle.org/browse/MDL-40855