вот твой файл https://bunker.zakviel.ru/js/bunker.js
вот из него js 
var isFullscreen = false;
document.getElementById('theatre').addEventListener("click", function(){
    if (isFullscreen) {
        closeFullscreen();
    } else {
        openFullscreen();
    }
    isFullscreen = !isFullscreen;
});
/* Close fullscreen */
function closeFullscreen() {
    if (document.exitFullscreen) {
        document.exitFullscreen();
    } else if (document.mozCancelFullScreen) { /* Firefox */
        document.mozCancelFullScreen();
    } else if (document.webkitExitFullscreen) { /* Chrome, Safari and Opera */
        document.webkitExitFullscreen();
    } else if (document.msExitFullscreen) { /* IE/Edge */
        document.msExitFullscreen();
    }
}
Кнопке дай 
id=theatre
и все