It seems to be the "default" style of some models for some reason.
Which makes me wonder if people already experimented with "style suggestions" to get different results: "Make it look like an 1998 GeoCities page" / 2005 Facebook / Newgrounds / DeviantArt / one of those Windows XP simulators with built-in window manager / etc
Other than that, great game!
considering free windows being light theme only, it should be a button, not a "system default"