para vc mudar o seu pano de fundo vc deve copiar o codigo css e colar de acordo com o fundo que vc escolher cada fundo tem a dica de qual themer vc deve usar ex:avack,Default Theme
Custom Theme
Oasis
Newspaper
Blue Slate
Blocks
Basecamp
Skyline vc deve escolher o themer que o fundo pede depois entra no codigo css
apague todo os codigos que estiver e cole o codigo que vc copiou do fundo que vc escolheu escolha o fundo da sua pagina no link acima ex:Eric Clapton's signature strat put on a theme. That and some circle brushes in the background and in the menu box. Enjoy!

para trocar o Theme:
1. Click no 'Customize My Site'
2. escolha theme pelo nome avlack, select it from 'Choose a Theme' first
3. Once your base theme is Avlack, click on 'Custom CSS'
4. Copy and paste the code below into the empy text box
5. Click "preview" to see how it looks on your site, and click "save" if you like it


body {
background: url(http://i152.photobucket.com/albums/s174/migrrr/greenstratbackg.jpg) repeat fixed top left;
}



/*TITLE*/
h1#page_owner_title {
color:#000000;
}
div.owner_nav {
background:transparent none no-repeat scroll 0pt -10px;
border-color:#000000;
border-style:none;
}
/*NAVIGATION BAR*/
div#subnavc {
background-color:transparent;
border-top:6px none #ffffff;
}
/*BODY*/
div#page_start {
border-color:#000000;
border-style:none;
}
td#maincontent {
background-color:transparent;
}
.itembox {
background:transparent none repeat scroll 0%;
border-color:#000000;
border-style:none;
color:#000000;
}
/*title box*/
.itemboxsub {
color:#000000;
}
/*Post Date (inside title boxez)*/
.itemsubsub {
color:#000000;
}
/*Post Date (inside boxez)*/
.posteddate {
color:#000000;
}
/*RIGHT RAIL*/
td.rail {
background-color:transparent;
}
.railstart {
background:transparent none no-repeat scroll -30px 0pt;
}
.railbody {
background:transparent none repeat scroll 0%;
border:none;
}
.railend {
background:transparent none repeat scroll 0%;
}
.railsep {
background:transparent none repeat scroll 0%;
}
/*FOOTER*/
div#ownedfooter {
background-color:transparent;
border-color:#000000;
border-style:none;
color:#000000;
}
/*VIEWING HISTORY*/
div.stats div.infobox .boxbody {
border:1px none #000000;
}
div.stats .infobox {
background-color:transparent;
}
div.stats {
color:#000000;
}
/*LINKS*/
a, a:visited, a:link {
color:#000000;
}
/*PHOTO ALBUM*/
div.prevnextlinks, div.prevnextlinks a, div.prevnextlinks a:link, div.prevnextlinks a:hover, div.prevnextlinks a:visited, div.prevnextlinks a:active {
background:transparent none repeat scroll 0%;
}
.album {
color:#000000;
}
/*Linear View*/
.itemboxalbum {
background:transparent none repeat scroll 0%;
border-color:#000000;
border-style:none;
border-width:1px;
color:#000000;
}
/*OTHER LINKS & TEXT*/
/*other pages - owner post administration*/
a.mine, a.mine:link, a.mine:visited {
color:#000000;
}
/*other pages - visitor post functions*/
div.ritemactions a, div.ritemactions a:link, div.ritemactions a:visited {
color:#000000;
}
/*other pages - owner reply administration*/
div.actionlinks a.mine, div.actionlinks a.mine:link, div.actionlinks a.mine:visited {
color:#000000;
}
/*other pages - navigation links*/
div.relatedlinks {
color:#000000;
}
/*deleted replies*/
.replydeleted {
color:#000000;
}
/*Tag:*/
div.relatedlinks {
color:#000000;
}
/*text for uploading videos from other sites*/
div.add_sublabel {
color:#000000;
}
/*links for uploading videos from other sites*/
div.add_sublabel a, div.add_sublabel a:link, div.add_sublabel a:visited {
color:#000000;
}
div.add_sublabel a:hover {
color:#000000;
}
/*REPLY BOXES*/
.replybox {
background:transparent none repeat-x scroll -25px 50%;
border-style:none;
color:#000000;
}
.replyboxread {
background:transparent none repeat-x scroll -25px;
border-color:#000000;
border-style:none;
border-width:1px medium medium;
color:#000000;
}
.replyboxstamp {
color:#000000;
}
.quotet {
background:transparent none repeat-y scroll -25px 50%;
border:1px none #000000;
color:#000000;
}
/*TABLE VIEW*/
/*border do not removed yet*/
.tablediv1 th {
background-color:transparent;
color:#000000;
}
.tablediv1 tr {
background-color:transparent;
}
.bodysummary {
color:#000000;
}
/*on mouse hover*/
.tablediv1 tr:hover {
background-color:transparent;
}

/*PAGE TOP*/
div.owner_nav {
border: none;
margin: 0 auto;
height: 222px;
width: 780px;
border: 0px solid #000;
border-bottom: none;
padding: 39px 0px 0px 0px;
background: url(http://i152.photobucket.com/albums/s174/migrrr/greenstrattop.jpg) no-repeat top center;
background-color: #000;
background-position: 0 0px;
}

/*PAGE TOP MENU*/
div#subnavc {
padding: 107px 0 0 0;
background-color: #000;
border: none;
width: 780px;
height: 40px;
background: url(http://i152.photobucket.com/albums/s174/migrrr/greenstratbottom.jpg) no-repeat bottom center;
}