Applying a global style, but exlcuding sites to which another style is applied?

[mattcelt]

I apologise if this has been answered before.. I tried to search before I posted, but I may not have the right terminology.

I'm trying to build a style that only applies if there are no other styles applied. So a sort of global default that works for any site that doesn't have a local style applied.

All of the "applies to" options in Stylus generally appear to be inclusive rather than exclusive ("match if", rather than "match except if"). Is there a way to detect that another style has been applied and prevent the global style from applying? (For instance, if I put a variable (--style-present, let's say) in :root in each site-specific style, do those become cross-CSS accessible to Stylus after applying them? Could I then look for the presence of that variable and use it?)

Thanks in advance!

[tophf]

CSS variables are applied to the actual DOM elements so yes you can use them across styles. As for exclusions, you can use a negative regexp lookahead.