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