So, first up: me personally (and all the projects I've worked on), I don't use ReactJS, Polymer or etclib. So I don't have any of that baggage (though I've got all the vanilla JavaScript baggage instead!).
I do tend to use Less on projects though, and ideally I would create component files for each "section", but honestly it's not really something I've had to do. Either because my projects are small when I've done all the CSS myself (i.e. my blog), or someone else will produce the CSS, and they've already broken it up into logical sections.
I think separate smaller files always makes sense no matter what you're doing, it's easier to consume, debug and test.