Rst0

"web_accessible_resources": [
      {
         "resources": [ "inner.js", "test.css" ],
         "matches": [ "https://www.youtube.com/*", "https://m.youtube.com/*" ],
         "extension_ids": []
      }
   ],

   "host_permissions": [
         "https://www.youtube.com/*",
   ],
и он работает

const injectJS = document.createElement('script');
   injectJS.type='text/javascript';
   injectJS.src=chrome.runtime.getURL("inner.js");
   document.body.appendChild(injectJS);