Increase indent of child menu items on mobile

By default the child menu items in the responsive (secondary) menu are indented only by the width of the added - (dash). I'd like to either increase the indent significantly (20px or more) or simply add additional dashes (4 or more) to visually offset the child menu items from their parent menu items. Can anyone help with this? I've scoured the responsive.css and style.css files as well as catchkathmandu-menus.php but haven't found where the code that creates the dash and the indent resides. Thanks!