This is my first time working with wordpress and I'm having the same issue. Did you have any luck with it Penelope?
I've scoured through the css and I'm still trying to figure it out. The css david.bailey refers to is:
margin: 0 auto;
Unfortunately it is getting the 50px from somewhere else. I've added min-width: 1100px which makes the site full width again but the tables and layout is still messed up.
Interestingly in the Mantra settings for content/sidebar width I'm seeing some strange text floating over the main text which reads:
Sidebar(s) = px
^——— Total width = 0px ———^
Sidebar(s) = %
^——— Total width = 0% ———^
I'm thinking these are supposed to be input fields but they are not, so it's just inserting 0 and then the 50px is being inherited from some other element. I'm also getting the same favicon issue that Penelope is having, with no input field. So I'm thinking the issues are connected, possibly a script error that isn't executing? I'm going to investigate further. Hope this helps anyone else working on it.