app.css

*{  
    font-family: 'Kolker Brush', cursive;
    box-sizing: border-box;
}

body.dark { 
    background-color: #3C4048;
    transition: background 0.6s linear;
}

body.dark h1{   
    color: #EEEEEE;
}

.header{    
    font-size: 59px;
    font-weight: 500;
    text-align: center;
    margin-top: 9rem;
}

.label{ 
    height: 49px;
    width: 109px;
    background-color: #3F3B6C;
    display: flex;
    border-radius: 50px;
    align-items: center;
    justify-content: space-between;
    padding: 5px;
    position: relative;
}

.main-content {
        display: flex;
        justify-content: center;
        align-items: center;
        margin-top: 9rem;
}

.fa-moon {  
    color: #EEEEEE;
    font-size: 23px;
    margin-left: 0.5rem;
}

.fa-sun {   
    color: #FD841F;
    font-size: 23px;
    margin-right: 0.5rem;
}

.ball { 
    background-color: #FFDDD2;
    position: absolute;
    height: 39px;
    width: 39px;
    top: 4px;
    left: 4px;
    border-radius: 50%;
    transition: transform 0.6s linear;
}

.checkbox{  
    opacity: 0;
    position: absolute;
}

.checkbox:checked + .label .ball{   
    transform: translateX(59px);
}