*{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}
body {
    background-image: url('https://pa1.narvii.com/6389/2b01eb0fbc7b6355b739f2e506fb24879e25af7a_00.gif') ;
    background-repeat: no-repeat;
    background-size: cover;
    background-position: center;
}
.banner{
    width: 100%;
    height: 100vh;
    background:rgba(0, 0, 0, .5);
    display:flex;
    justify-content:center;
    flex-direction: column;
}

#btn { 
    margin: auto ;
    margin-top: 300px;
    display: block;
    background: yellow;
    width:15rem;
    padding:15px;
    border-radius: 10px;
    font-size:20px;
    outline: none;
    cursor: pointer;
    font-family: 'Potta One', cursive;
}

@media(max-width: 764px){
       #btn {
           margin-top: 250px;
       }
}