Seems clear they are interested in the NeXTSTEP aesthetic in addition to the underlying framework.
Are you talking about Win95's "Maximize" control?
Compare the iconify button with the icons representing iconified windows below:
https://raw.githubusercontent.com/trunkmaster/nextspace/mast...
There are some similar ones mentioned in its Wikipedia article:
https://en.wikipedia.org/wiki/FVWM95
IceWM is quite themeable and is actively developed: