Change background color to black

Since it’s a commercial theme, you should try posting a comment on the theme’s page. But as a general suggestion, if you learn to use a web debugging tool like Firebug or Chrome Developer Tools, you can figure out what CSS rules you need to add to change your site’s appearance. If your theme has a custom CSS option, you can add your new CSS there, or you can create a child theme and add the CSS to your child theme’s style.css file, or use a CSS plugin to add your new CSS (easiest option).