I've been going insane trying to figure out WHAT css selectors to use to make the blue [expand] text be any other colour by default, from the mediawiki collapsible elements stuff https://www.mediawiki.org/wiki/Manual:Collapsible_elements.
on click it underlines and turns to the color its meant to be, but by default when not pressed nothing changes and I'm just lost on what more I can try.
I'm not the best at CSS and I generally get by with digging in inspect element to see how things are glued together but in this case no matter what I've tried to target it just doesn't budge. 
It's good to know you already tried using the inspector. Sometimes !important or more specific selectors are needed to overwrite the style (though in this case the selector just changed after some MediaWiki update). The inspector shows which style is taking effect at least.