I don't know about KWin, but there is a BeOS-like XFCE theme [1] so you might be able to swap KWin with XFWM and use that. [1] https://i.imgur.com/emP3Jk4.png

I don't know about KWin, but there is a BeOS-like XFCE theme [1] so you might be able to swap KWin with XFWM and use that. [1] https://i.imgur.com/emP3Jk4.png https://i.imgur.com/emP3Jk4.png (not my shot)