Another Wrapper Question

I’ve found many wrapper posts but none that have really helped me get things sorted out. I’m trying to add a wrapper around the main content on my site so it actually appears centered on the screen. I’m currently using the WooThemes Outlet theme, which is a child of Storefront. I’m new to CSS and trying to figure this out as I go so please forgive the entry-level question… but I’m looking for instructions (I do actually want to learn this stuff and have started going through tutorials at Code Academy and W3C) on how to actually add the wrapper (including defining the #wrapper ID in the HTML. Site is bare with just the template in place and can be found here: https://f64.452.myftpupload.com/

Thank you in advance for the help.