Что-то записываем в куки (и используем)

 
// Toggle dark mode, toggle menu mode... etc...
$(document).on('click', '#toggledark', function() {
    var mode = getCookie("dayNight");
    var d = new Date();
    d.setTime(d.getTime() + (365 * 24 * 60 * 60 * 1000)); //365 days
    var expires = "expires=" + d.toGMTString();
    if (mode == "dark") {
      document.cookie = "dayNight" + "=" + "light" + "; " + expires + ";path=/";
      document.getElementsByTagName('body')[0].classList.remove('dark');
    } else {
      document.cookie = "dayNight" + "=" + "dark" + "; " + expires + ";path=/";
      document.getElementsByTagName('body')[0].classList.add('dark');
    }
});
// TODO: move to util
function getCookie(cname) {
    var name = cname + "=";
    var ca = document.cookie.split(';');
    for (var i = 0; i < ca.length; i++) {
        var c = ca[i];
        while (c.charAt(0) == ' ') {
            c = c.substring(1);
        }
        if (c.indexOf(name) === 0) {
            return c.substring(name.length, c.length);
        }
    }
    return "";
};
 

 
.dark {
  background: #333;
}
 

 
if(\Request::getCookie('dayNight') == 'dark') { ...