Lisp can be very much declarative. It can be literally any paradigm by virtue of being a direct representation of it's AST while all other languages have to be parsed into AST.
CSS's cascading rules can be diagrammed as a tree, so can be shown as s-exprs.
I can clearly imagine how s-exprs can replace html, but I don't understand how s-exprs would replace css. Would you show a toy example?