title: Modify the browser UI using extensions?
body: <p>Is there an API to modify the general browser UI in Chrome extensions or WebExtensions? For example I would like to modify the tab bar so that it can display multiple rows of tabs without creating a toolbar that sits below the address bar. Or simply add some styling to the navigation bar etc.<br>
  I can't seem to find an API that would allow you to do anything like that.</p>