body{
    background-image:url(img/body_bg.gif);
    margin:0px;
    padding:0px;
}
/*今回は600*400のBOXを画面中央に配置*/
div#main{
    background-image:url(img/main_bg.jpg);
    overflow:auto;
    height:400px;
    width:600px;
    position:absolute;
    top:50%;
    left:50%;
    margin-top:-200px;
    margin-left:-300px;
}

あんまり使わないと思うけど。

position:absolute;
top:50%;
left:50%;
この３行で、#mainの左上を画面ど真ん中へ。

margin-top:-200px;
margin-left:-300px;
この2行で、600*400のBOXの半分ずつをマイナスして、
#mainの真ん中を画面の真ん中へ

/*---------------------------------------------

Default CSS
author:chibi

---------------------------------------------*/
/* important */

body {
  font:13px/1.231 arial,helvetica,clean,sans-serif;
  *font-size:small;
  *font:x-small;
}

table{
  font-size:inherit;font:100%;
}

pre,code,kbd,samp,tt{
  font-family:monospace;
  *font-size:108%;
  line-height:100%;
}

html {
  overflow-y: scroll;
}

*{
}

/* clearfix */
.clearfix:after {
   content: ".";
   display: block;
   clear: both;
   height: 0;
   visibility: hidden;
}
.clearfix { display: inline-block; }
/* exlude MacIE5 \*/
* html .clearfix { height: 1% }
.clearfix {display:block;}
/* end MacIE5 */

/* Reset */

body, div, h1, h2, h3, h4, h5, h6, p, dl, dt, dd, ul, ol, li,
brockquote, pre, table, caption, address,
form, fieldset, legend, object,img {
  margin: 0;
  padding: 0;
}

h5, h6, pre, table, code, kbd, samp, acronym, input, textarea {
  font-size: 100%;
}

a, ins {
  text-decoration: none;
}

address, em, cite, dfn, var {
  font-style: normal;
}

input, textarea {
  font-family: sans-serif;
  font-weight: normal;
}

img {
  vertical-align: baseline;
}

a img {
  border: none;
}

/* Setting */

body {
	line-height: 1.5;
	color: #333;
	background-color: #fff;
	font-family: Arial, Helvetica, sans-serif;
}

* html body { /* for IE6 */
  font-size: small;
}

*:first-child+html body { /* for IE7 */
  font-size: small;
}

ul {
  list-style-position: outside;
  list-style-type: none;
}

ol {
  list-style-position: outside;
}

table {
  border-collapse: collapse;
}

legend {
  color: #333;
}

div#main p,
div#main ol,
div#main dl,
div#main blockquote,
div#main pre,
 {
  margin: 3px 0;
}
.clear{
	clear:both;
}

/* Option *//*

* html .smallFont {
  font-family: 'メイリオ', 'ＭＳ Ｐゴシック', sans-serif;
}

*:first-child+html .smallFont {
  font-family: 'メイリオ', 'ＭＳ Ｐゴシック', sans-serif;
}

* html ul li dl,
* html ol li dl {
  display: inline;
}

*:first-child+html ul li dl,
*:first-child+html ol li dl {
  display: inline;
}

*/

/*---------------------------------------------

Font Size

20px : 154%
18px : 139%
16px : 124%
15px : 116%
14px : 108%
13px : 100%
12px :  93%
11px :  85%

---------------------------------------------*/

何回もいじってますが、未だに定まらない。
とりあえず、2008年版ということで。