It's a mockup I found on image search (from searching "neobrutalism GUI" or the like): https://www.magnific.com/premium-vector/hand-drawn-neo-brutalism-ui-elements-collection_186004756.htm

And yeah, that theme you linked is already pretty cool. Not terribly enamored with the retro aesthetic personally (especially with bad contrast like here), but if that can be done with KDE/Kvantum, then an actual neobrutalist theme, or just one with the papercuts fixed, is likely just as possible...