Custom CSS is part of WordPress' Customizer and you can add there custom CSS for any element on your site, even for Modula galleries. However, integrated in Modula's options there is a tab designed specifically to add CSS for each gallery. The CSS add will only apply to that gallery.
When do we use Custom CSS?
When we have no choice, when there is no option in the dashboard and we really want to hide, pronounce, color, or delimit certain elements.
In the example above you will see a custom CSS that change the images' opacity. This CSS will apply only to this gallery and no other.
If you want to add a custom CSS that will apply to all galleries, then add it in the WP admin dashboard > Appearance > Customize > Additional CSS.