Summary: You shouldn’t organise CSS by splitting it into separate files any more than you should organise a pane of glass by dropping it on a concrete floor.
Things To Avoid When Writing CSS

Though the metaphor is nice, I don’t think *not* splitting out CSS actually works in practice. When you’re working on a large site, a single CSS file would easily get thousands and thousands of lines long, which means you don’t have an easy way to know which styles you (or someone on your team) has written already, and you’re using a text find to figure out where to update in the file — if you can remember what you called your comment. It’s not about specificity, it’s about maintainability.