Platinum Equity, Beverly Hills-based private equity firm that owns the Union-Tribune, has bought Denver's Quark, a software company that revolutionized magazine and newspaper industries with page layout software for desktop computers. Under previous management, the U-T had been slow adapting this technology. Quark's software facilitates publication of content for tablet, web, mobile, and social media platforms. Quark has lost much of its market share to software maker Adobe Systems. Terms were not disclosed.