Can CSS be used with other than HTML documents?

Yes you can use it with XML

CSS can be used with XML, SVG and XUL.

CSS can be used anywhere you want in order to style your sheets with XUL,SVG,XML,JQUERY etc.

As Muhammad Majid Saleem answered, it's important to take into account that some SVG elements can be styled using CSS.
CSS is not just for styling HTML.
Moreover, CSS selectors are used by selector engines like Sizzle (used by jQuery), and the native DOM methods querySelector and querySelectorAll.
So, they are very important to be learned by developers and designers.