// description of your code here

function simpletogglediv(whichLayer)
{
         var theElementStyle = document.getElementById(whichLayer); 

        if(theElementStyle.style.display == "block")
        { 
                theElementStyle.style.display = "none";  
                 
        }
        else
        { 
                theElementStyle.style.display = "block"; 
        }
} 

You need to wrap in another div, and center via that one.

e.g.

body {
text-align: center;
}

.outer_div {
text-align: left;
width: 750px;
height: 500px;
background: white;
padding: 5px;
margin: 0 auto;
}