Note: We try our best to keep changes to minimum, but we can't guarantee that the gallery HTML code will stay the same. If we change the code, it's possible that any custom CSS you have will stop working. We also can only write custom CSS for paid users.
Here is a an example of custom css that hides sidebar in Covet.pics popup:
How to add CSS to your gallery:
1. directly in the liquid template - next to gallery embed code
3. directly into the Page HTML/CSS - if a gallery is getting embedded to a Page, same as the liquid template, the code can be added directly to the page - next to gallery embed code