Center Portfolio Gallery images and remove header background color transparency

how do I center the portfolio gallery content, I’ve edited each subsection to be centered but still some of the images seem to float to the left. thoughts?

also, is there a way to keep the header solid white (when scrolling) as a constant even at that top of the page. note how the logo is small until you scroll down a bit and the logo size increases. I’d like to keep it increased and solid white from the start.

sample page here: https://c63.76a.myftpupload.com/portfolio/clark/

Thanks!