style.css

[data-theme="light"] {
    --primary: #2a83d6;
    --background: #FFF;
    --text: #000;
}

[data-theme="dark"] {
    --primary: #e9833e;
    --background: #000;
    --text: #FFF;
}

body {
    color: var(--text);
    background-color: var(--background);
    font-family: sans-serif;
    margin: 0;
}

h1 {
    color: #FFF;
    margin: 0;
}

header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    background-color: var(--primary);
    padding: 20px 10px;
}

main {
    padding: 0 20px;
    display: flex;
    align-items: flex-end;
    flex-direction: column;
}

article {
    margin-top: 20px;
}

button {
    background-color: var(--primary);
    padding: 10px;
    border: none;
    font-family: sans-serif;
    color: #FFF;
    margin-top: 20px;
    font-weight: bold;
    border-radius: 5px;
}

.switch {
    position: relative;
    display: inline-block;
    width: 60px;
    height: 34px;
}

.switch input {
    opacity: 0;
    width: 0;
    height: 0;
}

.slider {
    position: absolute;
    cursor: pointer;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background-color: var(--background);
    -webkit-transition: .4s;
    transition: .4s;
    border-radius: 34px;
}

.slider:before {
    position: absolute;
    content: "";
    height: 26px;
    width: 26px;
    left: 4px;
    bottom: 4px;
    background-color: var(--primary);
    -webkit-transition: .4s;
    transition: .4s;
    border-radius: 50%;
}

input:cheked + .slider {
    background-color: var(--background);
}

input:checked + .slider:before {
    -webkit-transform: translateX(26px);
    -ms-transform: translateX(26px);
    transform: translateX(26px);
}