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Hmm, would you mind providing a mockup design of what should be implemented? The first two CSS are correct. Your site is being rendered differently on each screen, on my end with a large screen, space is bigger since the right most sidebar is floating. And it has two sidebars, one in content and another is floating.
Regretfully what you are trying to achieve is not possible with Icon stack.
It could be possible with custom development, but this would be outside the scope of support we can offer. You may wish to consult a developer to assist you with this. X is quite extensible with child themes, so there are plenty of possibilities. Thanks for understanding. Take care!