PARIS — Francesco Risso and his team were prepping for Marni’s Paris debut on the top floor of brand owner OTB Group’s French headquarters. “We couldn’t not have been in an attic in Paris, right?” Risso erupted with a gleeful cackle. In the space of three days, it had been transformed into an artist’s garret, walls and ceiling covered with vivid Van Gogh-like paintings of blue skies, sunbursts and flowers. “It was a bit too…