We have (finally) approved the Moodle CSS Coding Style:
https://docs.moodle.org/dev/CSS_Coding_Style
Many thanks to Frédéric Massart who did the bulk of the work getting these guidelines in place and to Daniel Wahl, Amy Groshek and others which this document was built on.
Shortly we will start linting css, less and scss with stylelint in grunt and ensure conformance to the style in Moodle core - see the issues in epic MDL-55058 for progress on that.
As this is now approved - please request changes to the style by filing a coding style bug in the tracker.
thanks!