Search for the line in the index.php:
PHP Code:
if ($cfgrow['markdown'] == 'T') {
require("includes/markdown.php");
}
add the following code below that code:
PHP Code:
if ($_SERVER['REMOTE_ADDR']=='127.0.0.0') // replace with your own address
{
$cfgrow['template']='templatefolder'; // replace with your template folder
}
Based on your IP it will change the template.