Using the CSS @import Rule
http://www.cssnewbie.com/css-import-rule/

Glazebrook, Rob L.
CSSnewbie
2008

Abstract:

Even the most complex style sheet starts out with a single rule. But when you’re working on a particularly massive and complex website, over time your style sheet will inevitably start to reflect the site’s size and complexity. And even if you employ every trick of organizing your CSS in the book, you might find that the sheer size of the file is simply overwhelming. At that point, you might want to consider splitting your style sheet up into several smaller CSS files. That’s when the @import rule can come in quite handy.