Shares of Mytheresa.com are set to begin public trading on Thursday. For the luxury e-tailer’s initial public offering, its owner and an existing shareholder, private-equity firm Ares Management, priced shares Wednesday night, raising $407 million for a valuation of $2.2 billion. Mytheresa’s IPO follows the public debut of Poshmark, the peer-to-peer fashion resale platform that completed…