I've tried setting padding and margin to 0 under BODY Box in the Post/Page Styling options, but all of my posts still seem to have about 15px of padding/margin on all sides.

All I really want to be able to do is adjust the padding on the left side so the entire post doesn't seem too indented relative to my header etc.

What's the best way to do this?

Thanks very much any help! I love this theme.

The space you're seeing there is probably caused by the padding of the parent container, the middle column, see A. Theme Options -> Center -> Center column style

padding: 10px 15px;

That worked perfectly. I'm going to make a donation now--I really appreciate all of your excellent work on this theme, and especially this kind of quick support.