@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}"