Can you use someone else's Style Sheet without permission?

This is a somewhat fuzzy issue. As with HTML tags, style sheet information is given using a special language syntax. Use of the language is not copyrighted, and the syntax itself does not convey any content - only rendering information.
It is not a great idea to reference an external style sheet on someone else's server. Doing this is like referencing an in-line image from someone else's server in your HTML document. This can end up overloading a server if too many pages all over the net reference the same item. It can't hurt to contact the author of a style sheet, if known, to discuss using the style sheet, but this may not be possible. In any case, a local copy should be created and used instead of referencing a remote copy.