Negative margins in Omega content zone
OK, you really have two options.

One is to put a more specific rule in your media query sheets.  So, you would need to place a rule in mytheme-alpha-default-wide.css  Normally, this comes before the grid sheet, so you either need a more specific rule or !important.

.zone-content .container-12 { width: 1300px !important; }


Your second option is to implement implement a hook_css_alter(), find the wide sheet for your grid system (eg, alpha-default-wide-24.css), and then swap it out with your own version.  Then, in your own version you can do something like:

.container-12 { margin-left: auto; margin-right: auto; width: 1200px; }
.zone-content .container-12 { width: 1300px; }


And, as a side note, you will likely have issues due to redefining the grid width.  The grid classes inside it will not work as expected.