style.css

.btn{
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  background: rgb(48, 48, 177);
  font-size: 20px;
  color: white;
  padding: 10px 30px;
  cursor: pointer;
}

#modalContainer {
    background-color:rgba(0, 0, 0, 0.3);
    position:absolute;
    width:100%;
    height:100%;
    top:0px;
    left:0px;
    z-index:10000;
}

#alertBox {
    position:relative;
    width:500px;
    min-height:100px;
    margin-top:250px;
    border:1px solid #666;
    background-color: pink;
    background-repeat:no-repeat;
    background-position:20px 30px;
}

#alertBox h1 {
    margin:0;
    font:bold 0.9em verdana,arial;
    background-color:white;
    color: black;
    border-bottom:1px solid #000;
    padding:2px 0 2px 5px;
}

#alertBox p {
    font:0.7em verdana,arial;
    height:50px;
    padding-left:5px;
    margin-left:55px;
}

#alertBox #closeBtn {
    display:block;
    position:relative;
    margin:5px auto;
    padding:7px;
    border:0 none;
    width:70px;
    font:0.7em verdana,arial;
    text-transform:uppercase;
    text-align:center;
    color:#FFF;
    background-color:#95b9d8;
    border-radius: 3px;
    text-decoration:none;
}