Custom CSS Editor not working

Hello, I'm using Catch Box Free Theme. I want to do some small modifications to the CSS, but the Custom CSS Editor doesn't show, meaning the box where i can insert my code is missing completely. I tried to logout und load the site again, but still the CSS Editor doesn't show. I don't have made any modifications to the theme, just ulpoaded a background and header image. What can I do to change the CSS? Thanks for help!