If you have skills when it comes to HTML and CSS, you can perform changes to your theme's design.
Adding CSS with the Bootstrap Darkroom theme
If your gallery uses the Bootstrap Darkroom theme, you can add custom CSS code in the theme's settings through the "Custom CSS" field.
Learn more about Bootstrap Darkoom
Adding CSS with LocalFiles Editor
The LocalFiles Editor plugin lets you edit Piwigo files directly from the administration.
For piwigo.com customers, this plugin only lets you add custom CSS code.
For users of a self-hosted Piwigo gallery, this plugin lets you edit other files (local configuration, language files, personal plugin). Learn more
Once the plugin has been activated, go to its settings.
You then have access to a window that lets you load the CSS files of the themes installed in your gallery. You can switch themes by using the drop-down menu and clicking on Edit.
By adding custom CSS code with this method, you are making sure that Piwigo's code doesn't get "crushed": therefore, it is the best way to proceed, even if you have access to Piwigo's files (through FTP).