How to center modal window on the screen using CSS

style.css

.container {
    position: absolute;
    padding: 0;
    margin: 0;
    left: 0; 
    top: 0;
    width: 100vw; /* margin: 0 and width: 100vw: 
                     100vw means the same as width:100% including the document margin, 
                     100vw is calculated relative to document layout including document margin, 
                     100% is calculated relative to parent layout */
    height: 100vh;
 }

.modal {
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translate(-50%,-50%);
}

.container-color {
    background-color: rgba(127,127,127,0.1);
}
.modal-color {
    -webkit-border-radius: 4px;
    -moz-border-radius: 4px;
    -ms-border-radius: 4px;
    -o-border-radius: 4px;
    border-radius: 4px;
    background: rgba(255, 255, 255, 1);
    -webkit-box-shadow: 0px 1px 5px 0px rgba(127, 127, 127, .35);
    -moz-box-shadow: 0px 1px 5px 0px rgba(127, 127, 127, .35);
    -ms-box-shadow: 0px 1px 5px 0px rgba(127, 127, 127, .35);
    -o-box-shadow: 0px 1px 5px 0px rgba(127, 127, 127, .35);
    box-shadow: 0px 1px 5px 0px rgba(127, 127, 127, .35);
}