I've found some threads on stylizing post content vs. page content, but that only affects stuff in the center column (div.post). Is there any way to actually stylize the stuff above that for just posts? On all posts, I want to hide the header image and shrink the layout margins.

The only way I can think of doing this is adding a conditional ID based on post v. page in the main index just outside of the container. Is there an easier way to do it?

Stye your site for the posts. Then in the 'CSS Inserts' options style things for the pages using 'body.page' as the prefix for each CSS selector.

This worked. I reworked all post-only elements as the default, then restored the originals item by item with body.page as the prefix. It's still a bit of a clunky solution, but a solution nonetheless and a good trick in my Ata back pocket. Thanks!