Pls Help: Centering Adjustable – Width Page Items

When I first put together collagegallery.org, I was asked to try to replicate a page that had been generated on a Mac, and being a bit of a novice to WordPress, I picked the closest WordPress template I could find and adjusted it a bit here and there.

The CSS wasn’t commented out though, and I’ve never been able to figure out how to get things centered on the front page for all screen sizes. They’re too far to the left when x>~1280, too far to the right when x<~1280

A recent change has made this flaw even more evident, so I turn to you, dear WP community, for assistance. it has something to do with how I’m using percentages to try and accommodate all screen sizes.

I’ve pasted the relevant (?) chunk of CSS, below. I’ve also pasted the full CSS here: https://pastebin.com/m9mVibtE

[CSS moderated as per the Forum Rules. Please just post a link to your site.]

Thanks for any pointers!