Add CSS to pages through a CSS file

Your manifest file must contain a content script pointing to a CSS file, that will load when a url matches your selected url fields…

"content_scripts": [ {
   "css": [ "scrollbars.css" ],
   "matches": [ "http://www.google.com/*" ]
} ],

This is the same method you would use to inject javascript code, except you would point to a js file instead…

More information here…