More Economical Use of Space

I really like the Argent theme except that I want the content area to be wider and the margins narrower everywhere. There is too much white space on the sides. Static pages, posts, text, images, everything are narrower than I want them with excessive space on both sides. I’ve seen prior questions directed to widening specific content but nothing that solves the overall problem. The issue is there on desktop with Windows and IE, but most noticeable on mobile where screen space is limited (e.g. iPhone iOS) … other similar sites and blogs just fill the screen better with less wasted space on the sides.

Along the same lines, I want less white space in text, a smaller line spacing. The large line spacing and wide margins cause my text to take up a disproportional amount of vertical space. Is there any way to globally tweak the line spacing?

Is there anything I can do to globally widen the content area and reduce the margin width?

The page I need help with: [log in to see the link]