Envira Gallery CSS Addon GPL Overview:
Do you want to customize your WordPress galleries?
Aside from our pre-made gallery templates, you can also use our CSS addon to add custom styles to your gallery display.
The CSS addon is a perfect, free utility addon to allow you to tailor customizations to specific galleries.
Once activated, a CSS box will be available under the Misc tab for your gallery where you can add your custom CSS styles. Information on how to style the gallery is available right there in the UI, and if you need more help, there is a dedicated doc piece for using the CSS addon.