Haven't tried playing around with it, but there is a setting in config/asgard/core/core.php:
'admin-theme' => 'AdminLTE'
You should be able make you own fork, (or even duplicate locally) and rename it, however you wont get the updates of the original theme. Also I think some of the plugins point to the adminLte.css file directly so it will probably be a hassle. I would try on a fresh install first before messing around on a real project.