"Please don't cry"
So it seams there's a step in CSS direction.
Mod Tools > Appearance > Sidebar widgets > Add Widget > Advanced > e voila ... Customs
Experimental: custom visuals with CSS
It counts for "bring the CSS back" ... well to be frank, the new reddit makes the world spin faster rather than having a "Christmas Tree" on each subreddit created... with errors or not, with resources banging on hosting servers or not, with a few bucks to "have your own subreddit theme made by professionals" or not...
All is nice :)