Las Vegas apparel company Zappos and Caesars Entertainment Corp. have partnered in rebranding Planet Hollywood’s concert venue. Beginning Monday, Axis theater is out. Zappos Theater is in. Jason Gastwirth, senior vice president of entertainment for Caesars, and Tyler Williams, “fungineer” for Zappos, described the agreement as the first step in a long-term relationship. “This is a strategic marketing plan and a coming together of our organizations,” Gastwirth over the phone Saturday. Zappos agreed to an exclusive, five-year marketing partnership with Caesars. The deal could lead to additional venue-branding possibilities between Zappos and Caesars. But for now, Williams said, “We are pouring everything we have into this partnership.”