The TemplateStyles extension introduces a <templatestyles> tag to specify that a stylesheet should be loaded from a wiki page. Placing this in a template allows the template to have custom styles without having to place them in MediaWiki:Common.css.
For installation and setup instructions, see https://www.mediawiki.org/wiki/Extension:TemplateStyles.
For instructions on how to use the extension as an editor on a wiki, see https://www.mediawiki.org/wiki/Help:TemplateStyles.