Using JavaScript you can get the window dimensions and then render the theme via php based on the dimensions of the window. So if the patcher is viewing the website in a 400px x 400px screen you can do something like the following.
set this near the top of the index.php
if(!isset($_GET('r')) {
echo "<script language=\"JavaScript\">
<!--
document.location=\"$PHP_SELF?r=1&width=\"+screen.width+\"&Height=\"+screen.height;
//-->
</script>";
}
else {
if(isset($_GET['width']) && isset($_GET['Height'])) {
$width = $_GET['width'];
$height = $_GET['Height'];
}
}
then set this down where you set the session info in the index.php
if($width == 400 && $height == 400) {
Flux::config('ThemeName', array('bootstrap'));
}
Again, this assumes the patcher window is 400px x 400 px. Also note my php is a little rusty, so this may not be 100% but it should put you on the right track