var menu = document.querySelector('body.single-news');
var button = document.querySelector('button#skin-btn');
button.addEventListener('click', function() {
    var open = menu.classList.contains('light-skin');
    menu.classList.toggle('light-skin');
});

localStorage.setItem("myVariableName", "myVariableValue");

localStorage.myVariableName = "myVariableValue";

var x = localStorage.getItem("myVariableName");

var x = localStorage.myVariableName;