I’ve been an Alicia Silverstone fan since I first saw her serve ‘90s looks in the nostalgia-inducing Clueless and, now, she’s looking even more fabulous celebrating her 50th birthday, thanks to designer Christian Siriano. The Bugonia actor go way back with Siriana, so I fully expected him to give her something epic but clearly he’s taking notes from Halle Berry on how to celebrate major milestones. Silverstone’s b-day was marked with a post in w…