style.css

@import url('https://fonts.googleapis.com/css?family=Poppins:300,400,900&display=swap');
html {
    width: 100%;
    height: 100vh;

    display: flex;
        flex-direction: column;
        justify-content: center;
        align-items: center;
    background: var(--lightBg);
    font-size: 3vh;
    text-align: center;

    --lightBtn: #FE016C;
    --lightBg: #fff;
    --lightColor: #232323;
}

body {
    display: flex;
    justify-content: center;
    align-items: center;
}
html[data-theme='dark'] {
    background: var(--lightBg);
    --lightBtn: #FFBD07;
    --lightBg: #232323;
    --lightColor: #fff;
}
h1 {
    font-family: 'Poppins', sans-serif;
    font-weight: 300;
    color: var(--lightColor);
}

input[type=checkbox] {
    height: 0;
    width: 0;
    visibility: hidden;
}

label {
    cursor: pointer;
    text-indent: -9999px;
    width: 90px;
    height: 50px;
    background: var(--lightBtn);
    margin: 0 auto;
    display: flex;
    justify-content: center;
    align-items: center;
    -webkit-border-radius: 100px;
    -moz-border-radius: 100px;
    border-radius: 100px;
    position: relative;
}

label:after {
    content: '';
    background: #fff;
    width: 40px;
    height: 40px;
    -webkit-border-radius: 50%;
    -moz-border-radius: 50%;
    border-radius: 50%;
    position: absolute;
    top: 5px;
    left: 4px;
    transition: cubic-bezier(0.68, -0.55, 0.27, 01.55) 320ms;
}

input:checked + label {
    background: #FFBD07;
}

input:checked + label:after {
    left: calc(100% - 5px);
    -webkit-transform: translateX(-100%);
    -moz-transform: translateX(-100%);
    -ms-transform: translateX(-100%);
    -o-transform: translateX(-100%);
    transform: translateX(-100%);
}

html.transition,
html.transition *,
html.transition *:before,
html.transition *:after {
    transition: cubic-bezier(0.68, -0.55, 0.27, 01.55) 420ms!important;
    transition-delay: 0!important;
}