body {
    background-color: black;
}

.btn {
    padding: 5px;
    background: rgb(0, 0, 0);
    border: 1px solid rgb(255, 255, 255);
    border-radius: 2px;
    outline: none;
    width:250px;
    height:40px;
    font-size:15px;
    color: white;
} 

div {
    position: fixed;
    top: 50%;
    left: 40%;
    margin-top: -20px;
    margin-left: -125px;
}