Enfold is a very popular theme for WordPress. It also offers in its form designer a template for videos. You can even simply just enter the Youtube URL in a textblock and Enfold will detect that it belongs to video. Unfortunately often the fullscreen option is greyed out in the video panel. In internet you find many tricks like adding an ifram tag or css styles. For me nothing worked with a good reliability.
But there is a solution. Simply use the plugin “Youtube embed”. There you will find an option for “Fullscreen” mode under “Profiles”: A button will allow the viewer to watch the video fullscreen. Additionally you want to keep a responsive design with automatic resizing: under “Profiles” enable “Responsive output”: Show full width and resize with the browser.
But don’t forget that plugins can make your website more vulnerable.