Continuing the discussion from Discourse Doc Categories:
I don’t know if there’s an overall reference, but the browser’s Inspect tool can help dig them up: Making custom CSS changes on your site: Finding the right CSS selectors
Some selectors are listed in A little help with a CSS selector?:
is-staffis-adminis-moderatoris-new-usergroup--primary_group_name
And the CSS Classes for Current User's Groups component gives the <body> tag classes for the logged-in user that you could target, like:
class="chat-enabled group-trust_level_0 group-trust_level_1 group-trust_level_2 group-trust_level_3" ... etc.