The window manager deals only in window terms, not in widgets, so you are fine. And one program can affect another`s windows and properties on them, so you can do this. In short, you need to use XChangeProperty() to affect the appropriate property and set the value appropriately. The trick, though, is that you typically don`t deal with these values directly. For a hint, use "xprop" on the window or another window to see the private property -- and now I forget the name, but it is something along the lines of _WM_MWM_DECORATIONS -- and combine that information with the man pages that describe XmNmwmDecorations.