Widen Main Content Area

Is there a way to widen the main content area in this theme without increasing the columns? I want to make my post area a little wider without changing the size of my sidebar. I’ve tried increasing the columns, but then it makes my homepage look super cluttered and my sidebar too small, so I want to keep it at 3 columns total.

Does anyone know the code to widen the content area manually? I’ve tried different variations of this:

#header, #main, #content-bottom, #breadcrumb, #footer {
width: 1120px;
}

#header, #main, #content-bottom, #breadcrumb, #footer {
margin-left: -90px;
}

And it just makes the whole page wonky without changing the width of the main content area.

Thank you!

http://www.welcometoerinsworld.com