Write karax code that generates a dom, add classes and id's etc. to your elements , then you use css to style the dom elements.
you load the css in the html (not from karax), you could if you really like create a link element from karax, but i would just write it in the html , then when the dom gets rendered by karax the style is applied.
Like enthus1ast, I assumed it was just a matter of adding the tachyons class names to whatever file you'd compile to js. Provided you include the tachyons CSS, it should work.
That said, I haven't actually tried this myself. So I'm interested in seeing the results. I'm a fan of both tachyons and nim.