Omeka + HTML/CSS/JavaScript

Hi, I’m new to Omeka, and was tasked with developing on top of a pre-existing site. I’ve been trying to figure out how I can develop on top of Omeka using HTML/CSS/JS, but haven’t had much luck so far, and was looking for some advice. I’ve never really worked with CMSes before, so I’m pretty clueless. Would doing something like this involve developing a theme/using PHP? Thanks!

It depends what you want to do.

Making or editing a theme lets you do lots of things with HTML/CSS/JS, but there are ways to use it in limited ways without doing that, like using Simple Pages.