IceWM is a free and open-source, lightweight, stacking window manager for X11. It is written in C++ and is designed to be easily customizable. It is also used by lightweight, beginner-friendly distributions like Puppy Linux.
USE flags for x11-wm/icewm Ice Window Manager with Themes
Install x11-wm/icewm with:
emerge --ask x11-wm/icewm