I’ve taken a really different approach with CSS. That won’t help you if you want to use a pre-defined one like from Bootstrap, but on the long run I think it will be better.
I simply dropped all CSS files and written an - admittedly lacking - implementation of JSS in pure Kotlin.
It lets me use themes easily, have the class names suggested by the editor, no CSS files to carry around in a 5 project deep dependency tree.
Not to mention WebPack dev server actually refreshes when I change something. 
The base class that does the CSS building: https://github.com/spxbhuhb/zakadabar-stack/blob/master/src/jsMain/kotlin/zakadabar/stack/frontend/util/css.kt
An example how I use it: https://github.com/spxbhuhb/zakadabar-samples/tree/master/01-beginner/my-life-my-theme-my-style