The best solution came from GitHub: https://github.com/lokesh/lightbox2/issues/172#issuecomment-228747592

#lightboxOverlay { position: fixed !important; top: 0; left: 0; height: 100% !important; width: 100% !important; }

#lightbox { position: fixed !important; top: 50% !important; transform: translateY(-50%); }