Photo gallery CSS
 

From the documentation:

Change the style of a photo gallery
The two default styles use exactly the same HTML, but different stylesheets. You can create your own CSS stylesheet, and use that with the photo gallery, if you prefer.

If there is a file photogallery.css in the same directory as the photos, then it will be used to display the gallery page.

UPDATE: Using my own CSS file is working fine after some CSS education.

Is that in the old classic UI or the current one

I use the current UI. But the classic Files UI, where you create the website gallery and upload photos is very similar (if not the same). So I would think the CSS replacement would work in either.

The stylesheet used by the photo gallery has nothing to do with the webmail interface.

- Bruce

I wanted to know if the CSS will work in the current UI and it will.

In the old UI we could have a CSS file and that change the fastmail webmail interface,