[Plugin: NextGEN Gallery] Displaying thumbnails as examples

Hi – i was looking at the template exapmles for Nextgen here: https://nextgen-gallery.com/templates/ and i wat to modify my template so it has cut corners as per the example here: https://www.webdesignerwall.com/tutorials/css-decorative-gallery/ 3rd row 1st pic. But i am unsure what to do and where – is there any tutorials etc on how to do this and what files need creating and putting where as its all a bit confusing? Thanks

https://www.ads-software.com/extend/plugins/nextgen-gallery/