Code/flask_project/pro_name/static/css/style.css

html,body,main { 
    height: 100%;
    width: 100%;
    margin:0;
    padding: 0;
    font-size: 25px;
}
body {
    background-image: url('/static/img/183201.jpg');
    background-position: no-repeat;
    background-size:100%;
}
#login {
    border: 1px solid white;
    border-radius: 5%;
    width: 30%;
    height: 300px;
    background-color: white;
    position: relative;
    left: 35%;
    top: 30%;
    text-align: center;
    opacity: 0.5;
}
table{
    width: 100%;
    margin-top: 10%;
}
tr,td{
    height:60px;
    text-align: center; 
}
input{
    height:25px;
    width: 80%;
}

.btn{
    height:35px;
    width:55%;
    font-size:22px;
    border:0px;
    border-radius:3px;
    font-weight: bold;
    background-color:  #428bca;
    color:white;
}