FvwmIconMan and TaskBar

Simple: You can’t. (I am assuming you mean in FvwmIconMan per se.)

– Thomas Adam

I was looking for the same effect, and found that setting “*FvwmIconMan: ReliefThickness” to 1 and then in my Colorset setting the “hi” and “sh” colors to the same thing, results in a satisfactory separation without the “3D” effect, for me at least, you may like the look of “ReliefThickness 2”. Hope that helps.