Boulder-based LogRhythm said Thursday that it sold off a majority stake to Thoma Bravo, a private equity firm with offices in San Francisco and Chicago. LogRhythm, which helps clients automatically monitor their computer systems for security threats, said the deal is expected to close by September. Financial terms were not disclosed. “We were looking for a partner that would have deep expertise in security and software, and Thoma Bravo’s only focus is software, and they have security as well,” said CEO and co-founder Andy Grolnick.