I found a way to add Magnific Popup (which does the image overlay like Lightbox does) to get the result I was looking for. It does not require you to add attributes to the link like Lightbox does.
In case it is helpful to anyone else:
I am using the “default” theme
Here is the Magnific Popup site Magnific Popup Documentation
Then I added the file paths to this file in my theme view>layout> layout.phtml (using the default.js line that is already in the file as a guide for how to format it)
The jquery is already linked - but i did have to move it to the bottom of the list of files in the layout.phtml to make sure it loads before the magnific popup js file.
Then I added the script to initialize the popup using the “from a group of elements with one parent” method. I put this directly in the item show file - view>omeka>site>item>show.phtml
I used the class name media-render where the documentation has parent-container like this:
delegate: 'a', // child items selector, by clicking on it popup will open
// other options
I think there is likely a better place to put this or way to include it? - if anyone knows would love to hear.
Now on my item show pages when you click the image file it opens in an overlay with a close button at top right.