.form-group {
    display: block ;
    margin : auto ;
    max-width: 35vw ;
}
/* 輸入完時 */
.form-control {
    color: white ;
}
/* 在輸入時 */
.form-control:focus {
    color: white ;
}
#er {
    display : block ;
    margin : auto ;
    height: 25vh;
}
#lg {
    height: 1vw;
    min-height: 15px ;
    color: white ;
}
#w {
    height : calc(15px + 0.5vw) ;
    color: white ;
}
#number {
    height : calc(20px + 1vw) ;
    color: white ;
}
body {
    background-image: url("背景.png") ;
    background-size: cover ;
}
#btn {
    display: none ;
}
@media only screen and (max-width: 800px) {
    body {
        background-image: url("手機登入背景.png") ;
        background-size: cover ;
    }#er {
        display : block ;
        margin : auto ;
        height: 10vh;
    }
    .form-group {
        background-color: rgba(82,65,123,0.52) ;
        height: 65vh ;
        display: block ;
        margin : auto ;
        max-width: 80vw ;
        padding: 18vh 5vw 18vh ;
    }
    #btn {
        display: block ;
        margin : auto ;
        padding: 1vh 5vw ;
        background-color: rgb(255,108,183) ;
        color: white ;
        border: none ;
        border-radius: 5vw;
        right : 1vw ;
    }
}