Is it possible to include one CSS file in another?

Yes:

@import url("base.css");

Note:

• The @import rule must precede all other rules (except @charset).
• Additional @import statements require additional server requests. As an alternative, concatenate all CSS into one file to avoid multiple HTTP requests. For example, copy the contents of base.css and special.css into base-special.css and reference only base-special.css.