css containers

example for your .css file

.center_div
{
border:5px dashed black;
margin-left:auto;
margin-right:auto;
margin-top:200px;
width:90%;
opacity: .9;
height:340px;
background-color:#d0f0f6;
text-align:left;
padding:38px;
}
and for your html page:  in < > but blogger will not allow them:

div class="center_div"

/div