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

Beautifully put, however insane the idea.

How does splitting CSS into files cause any problems at all? Surely it will just make it easier to know what CSS is responsible for what and therefore easier to maintain & build upon?

Not once have I thought ‘I wish this was all just one big CSS file’.