ModernLightbox

FandomizedLightbox, which is also called "FandomizedFileViewer", is a script that modernizes the lightbox/file viewer. It includes a dark theme, full screen and general modern styles.

Code
Add the following line in your wiki's Wikia.css. It must be added at the very top of the page, otherwise, it won't work. Alternatively, you can copy the contents in MediaWiki:FandomizedLightbox/code.css.