How to mass “aligncenter” images?

Hi everyone!

I’ve been blogging for 20 years so I have a lot of old content. I used to post smaller images and I don’t think I cared much about alignment back then.

I changed theme recently and I’m wondering if there’s a way to mass aligncenter images (css?).

Exemple of an old post with images aligned left: https://correresmidestino.com/no-power-in-pyongyang-i-mean-shenyang/ or https://correresmidestino.com/a-medieval-day-in-angers/

Exemple of a newer post, gallery with images aligned center: https://correresmidestino.com/toronto-is-nonthematic-which-makes-it-fascinating/

I noted that even if I center images manually in the editor, they still stick to the left side of the page.

Thank you for your input!

The page I need help with: [log in to see the link]