I like garden but I don’t have the impression it’s used that much in practice. My impression is companies use whatever is popular in the frontend world for their CSS, combined with node based tooling.
Learning reagent/re-frame is a solid investment. These or some variant thereof seem to be the most popular options for cljs based UI stuff.
Also keep in mind there’s often a distinction between people doing frontend development and people working on design and CSS. You should know enough CSS to get around, but to know the ins and outs is really a specialization in itself.