We (I) split ours into relevant sections so as to not call files that aren't needed e.g. we have a main.css which covers content presentation and is common to all pages, a secondary.css to handle navigation which is unique to the section of the site that you are on (4 main sections). A another.css is used for some pages that require styling that isn't relevant anywhere else.
We also have a large number of people that license our content and have a main.css file for them as well as an extra.css that sits at the end of the list to override some of our content styles as applicable to better match their own content (alternative fonts, link stylings and the like).
(Above namings are not actual)