Centering post within post grid

Hello! I’m sure this answer is out there somewhere, but I can’t seem to find it… By default, all “posts” modules display the post in left-aligned columns. I want to center them so the last few posts show up in the center of the page, not at the left.

In this example, Robin and Megan should appear in the center, not on the left.

I’ve tried just about everything I can think of in both module settings and custom CSS. I’ve used BB “posts,” Ultimate Addon “advanced posts,” and Power Pack “content grid.” What am I missing??