I am sorry about the delays. My fault entirely. I apologize. The code that Ismael gave you needs to be pasted in the admin area of the theme, in Coherence Theme Options > Styling and on the bottom there's the Quick CSS textbox. Please hit the save button and that's it.
I wouldn't put much weight into the WC3, or their buggy theme checking gimmick. Until HTML5 is finalized in the next few years, every browser has their own standards, so nobody knows exactly what it is that WC3 checks. Ebay, Amazon and Yahoo - each fails validation with over 300-500 errors according to WC3.
However looking at the PR of websites using Kriesi themes, it becomes self evident that Google approves and rewards the sites using those themes - which adds up to better traffic and more sales.
I am sorry, I don't have a wide screen Mac (though sure would love to get one) to test the site.
Great care must be taken when customizing a sophisticated responsive design since unlike regular themes, responsive themes are perfectly balanced to simultaneously work on home computers as well as mobile devices of various screen sizes, resolutions, capabilities and operating systems made by competing companies unwilling to conform to any standards.
Yet the payoff for that extra time spent dealing with a responsive design are huge, since 40% of online traffic today comes from mobile devices. No worries, the support team will do our best to ensure that you aren't abandoned, or I wouldn't be writing this at 7am on a nice Sunday morning. :)