I guess you could look for themes (Gnome: https://www.gnome-look.org/p/1143497| https://www.gnome-look.org/p/1143497| XFCE: https://www.xfce-look.org/p/1013908/

I guess you could look for themes (Gnome: https://www.gnome-look.org/p/1143497| https://www.gnome-look.org/p/1143497| XFCE: https://www.xfce-look.org/p/1013908/ https://www.xfce-look.org/p/1013908/) but you won't get the same unified GUI as Haiku has, since everything is built with unification in mind, compared to the Linux ecosystem where modularity is usually king. I guess Elementary OS is as close to a unified GUI you come that runs with a Linux kernel, but it tries to look more like macOS than BeOS.