Dark mod aktif etme.

                        <div class="checkbox checkbox-slider--b-flat">
                            <label class="mode-control">
                                <input id="mode-btn" type="checkbox">
                                <span>Dark mod</span>
                            </label>
                        </div>
<style>
    body
    {
        background-color: #eee;
        color: #222;
        transition: 300ms;
    }

    body.dark
    {
        background-color: #413d3e;
        color: #fff;
    }

    .mode-control span:nth-of-type(2)
    {
        display: none;
    }

    .mode-control input:checked + span
    {
        display: inline-block;
    }
</style>
<script src="script.js"></script>

document.getElementById('mode-btn').addEventListener('click', () =>{
    document.body.classList.toggle('dark');
    localStorage.setItem('mode', document.body.classList);
})

if (localStorage.getItem('mode') != ''){
    document.body.classList.add(localStorage.getItem('mode'));
    document.getElementById('mode-btn').checked = true;
}