Don’t rely on using CSS classes from a specific theme. It won’t work on a lot of other themes.
Our approach with theme extensions is to use basic HTML elements (like button, p, a, label etc.) which should pull styling on most themes to appear somewhat consistent, and then we apply our own custom CSS for some base styles/overrides as needed.
It’s never perfect, and can never match their theme guaranteed, but you can get close with this approach I’ve found, and then fix up anything extra with custom CSS.
I’d also recommend avoiding CSS/UI libraries if you care about your merchant and their customer’s experience; it’ll add a lot of extra code that’s unnecessary. If possible, write your own styles and build your own simple and clean components.
One other tip - if adding your own custom CSS, make sure you target your own custom CSS classes (and make them unique). So you avoid conflicting with other elements on their store, create unique classes for your extension only and target those.