theme.js

// credit AdminLTE, original code with css here: https://github.com/ColorlibHQ/AdminLTE/commit/e7ed74d4eae67cf00d6b1be6cbbe5f43e551c7b6

var toggleSwitch = document.querySelector('.theme-switch input[type="checkbox"]');
var currentTheme = localStorage.getItem('theme');
var mainHeader = document.querySelector('.main-header');

if (currentTheme) {
    if (currentTheme === 'dark') {
        if (!document.body.classList.contains('dark-mode')) {
            document.body.classList.add("dark-mode");
        }
        toggleSwitch.checked = true;
    }
}

function switchTheme(e) {
    if (e.target.checked) {
        if (!document.body.classList.contains('dark-mode')) {
            document.body.classList.add("dark-mode");
        }
        localStorage.setItem('theme', 'dark');
    } else {
        if (document.body.classList.contains('dark-mode')) {
            document.body.classList.remove("dark-mode");
        }
        localStorage.setItem('theme', 'light');
    }
}

toggleSwitch.addEventListener('change', switchTheme, false);