How to Set Padding around "Body" of Posts / Cut Indent?

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.