change font in MN Flow theme

I like this theme however I want to change the title font and the menu font. Can you tell which file I need to update it in? The style.css is empty and there’s no font defined there.
The google font style sheet given in Page source is <link rel=’stylesheet’ id=’open-sans-css’ href=’//fonts.googleapis.com/css?family=Open+Sans%3A300italic%2C400italic%2C600italic%2C300%2C400%2C600&subset=latin%2Clatin-ext&ver=3.9.1′ type=’text/css’ media=’all’ />. I can’t find the file where to change this font. Please help.