Coppermine Photo Gallery is a popular photo gallery script used all over the web. It has a theme preview feature that you can utilize when changing your gallery’s theme that allows you to view your gallery with the theme without activating it for your visitors. This allows you to preview the theme and see any errors or change any display settings.
To preview the theme enter your gallery URL’s homepage and add “index.php?theme=themefoldername”
for example; if your theme’s folder name in the coppermine themes directory is “newtheme”:
https://website.com/gallery/index.php?theme=newtheme

To change your view back to see the current active theme that your visitors see; instead of putting in the theme folder name, just put in “default”:
https://website.com/gallery/index.php?theme=default