[qusaze]

let html = document.querySelector('html');
 
window.addEventListener('unload',() => {
    localStorage.setItem('night', html.classList.contains('night') ? 1 : 0);
});
 
if (localStorage.getItem('night') == 1) {
    html.classList.add("night");
}
 
document.querySelector('.block').addEventListener('click',function() {
    html.classList.toggle("night");
});

Вот решение. Спасибо