[Theme: Twentyeleven] Adjusting Margins

Hi,
I’m trying to adjust the margins on the TwentyEleven theme (i.e. I want them smaller).

I found a guide here: https://zeaks.org/twenty-eleven-change-sidebar-width/ and that works BUT as I am using the Showcase template for my front page it buggers it up – the featured post width becomes squashed.

Any ideas?