(note: no use of float:left)
#outer {
margin:auto;
border: 0px solid #73AD21;
text-align:center;
width:100%;
}

#box {
border-radius: 15px;
background: #f1f1f1;
padding: 20px;
margin:auto;
width: 33%;
height: 300px;
display:inline-block;
}

pop-up
https://www.w3schools.com/cssref/tryit.asp?filename=trycss3_target_modal

CSS Elements
https://www.w3schools.com/cssref/css_selectors.asp