Easiest way is:
create some templates (eg: yellow, red)
create some homepages (eg: home_yellow, home_red)
link every homepage to it's own template
Place in every index.php from each template a pulldownmenu like:
HEAD:
<SCRIPT TYPE="text/javascript" LANGUAGE="JavaScript">
<!--
function openURI() {
var control = document.name.templates;
if (control.options[control.selectedIndex].value != '') {
location.href = control.options[control.selectedIndex].value;
}
}
//-->
</SCRIPT>
BODY:
<FORM ACTION="" NAME="name">
<SELECT NAME="templates" onchange="openURI()">
<OPTION VALUE="home_yellow.php" selected="selected">Template 1</OPTION>
<OPTION VALUE="home_red.php">Template 2</OPTION>
</SELECT>
</FORM>
Don't forget to change the SELECTED into the template that is show at that moment.