@niamu: here is the official answer from noprompt.

(require '[garden.selectors :as s])
(require '[garden.core :as css])

(css/css
  [(s/body s/after)
   {:background "blue"}])
;; => "body::after {\n  background: blue;\n}"

Looking at the documentation, that makes much more sense now.

Slowly getting the hang of this CSS stuff (css [((s/body (s/attr :data-tooltip)) s/after) {:color 'blue}]) =>"body[data-tooltip]::after {\n color: blue;\n}"