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.

The @import url("base.css"); works fine but bear in mind that every @import statement is a new request to the server. This might not be a problem for you, but when optimal performance is required you should avoid the @import.

Yes. Importing CSS file into another CSS file is possible.

It must be the first rule in the style sheet using the @import rule.

@import "mystyle.css";
@import url("mystyle.css");

The only caveat is that older web browsers will not support it. In fact, this is one of the CSS 'hack' to hide CSS styles from older browsers.

Refer to this list for browser support.