This is not a "CSS-in-JS" library.
The sole purpose of this solution is to allow you to conditionally apply styles to child elements, inside a containing element.
You can still use your regular CSS processing tool - be that PostCSS, SASS or LESS - to process your CSS files, and when certain styles are tied to a @container query with a certain condition, then it's the JS runtime that'll decide when to apply your them to the applicable elements.
As I'm not using any of those libraries, I'm not exactly sure how the setup would look like, but it's quite possible to apply your regular / static styles with your library of choice, and then use the meta builder to build the meta object containing the styles that need to be applied runtime.
Feeding such a meta object to the runtime is already what we're doing, the only difference being that this meta object is being conveniently generated for us by the postcss-plugin.
Looking into the tests for the meta builder might help you figure out how exactly this could be implemented.
Don't hesitate to post in issues if you feel like you have a good solution!