This callback triggered before theme is deleted.

<?php
function my_component_init(){
		ossn_register_callback('theme', 'before:delete', 'my_component_cb');
}
function my_component_cb($callback, $type, $params){
		//$params['theme'] contains the ID of the theme
}
ossn_register_callback('ossn', 'init', 'my_component_init');