I accept that's possible - if not likely (and everyone steals from each other!) - but even-so it only amounts to to the gunmetal-grey default colours and use of a 1px bevel/inset effect; because NS and NT3/NT4's UX/UI design and concepts are just so different otherwise.
...but I'm not personally convinced: instead, consider the demonstrable fact that similar engineering teams, working on similar problems, will independently come to substantially similar solutions; my favourite example to point to is how eerily-similar the Eurofighter Typhoon, Saab Gripen, and Dassault Rafale all look - even entirely indistinguishable at an air-show in-person - despite having zero shared pedigree - therefore it's possible that - given the constraints of desktop graphics hardware of the late-1980s/early-1990s - that a user-friendly desktop UI built around the concept of floating application windows - will all be similar in one way or another.
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My pet-theory for why that "Windows 95 1px bevel" look is so prevalent is because it suits working with premade UI graphics rasters/bitmaps using indexed-colors: for example, imagine a Windows-style Property Sheet dialog: prior to Windows 95, software would manually draw all of the elements of that dialog directly to the framebuffer (i.e. using unbuffered graphics) which was slow - ugly - and is the cmputer-equivalent of using a lavatory in a cramped bathroom actively undergoing renovations without any drywall/plastering). Even if there was enough vram for double-buffering it's still going to be slow: painting each and every button, checkbox (with the checkmark!) and tab header. So instead, many individual UI graphics elements could be prerendered (at design-time, hopefully by an actual artist), but not as single bitmaps for the entire dialog - but as an indexed color bitmap for each control type, so no slow/expensive draw/painting is required: only a simple blitbit for each checkbox, for example. Using an indexed-color bitmap based on a 4 or 8 colors palette (face, 3D light, 3D dark, transparent/BG; etc) means a single blob only a few hundred bytes in size can represent a chisel-cut bevelled checkbox - while integrating with whatever the user's preferred color scheme is.
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....of course now we'll just build a UI in Electron, to hell with memory usage or integrating with the user's OS appearance settings. Le sigh.