open: themes/default/style.min.css

find:

body{color:#000;background:#f9f9f9;margin:0;padding:0;font-family:verdana,arial,sans-serif;font-size:100.01%;}

replace with:

body{color:#000;background:#f9f9f9;margin:0;padding:0;font-family:verdana,arial,sans-serif;font-size:100.01%;width:800px;margin-left:auto;margin-right:auto;}