returning to theme default settings

Hi. I was just trying to customise my new site (www.invisiblecities.com.au – Matheson theme) using the ‘Customise’ options – no fancy HTML or CSS – and I have no idea what I did, but it’s pretty strange looking now. I’d like to get it back to the theme’s default settings, or just widen that white bit. Can anyone have a look and tell me what I did wrong and how I can fix it?
Thanks!
Asha