This is not the solution I had in mind but nevertheless interesting.
Colorset 10 fg #ffffff, bg #2b4e5e, hi #aaaaaa, sh #999999, Plain, NoShape
# Transparent
Colorset 16 Transparent
DestroyModuleConfig SimpleButtons: *
*SimpleButtons: Geometry 600x50+680+0
*SimpleButtons: Colorset 16
*Simplebuttons: Frame 0
*SimpleButtons: Rows 1
*SimpleButtons: Columns 13
*SimpleButtons: (1x1, Icon $[sbicons]/decor/squre-32x32.png)
*SimpleButtons: (1x1, Icon $[sbicons]/xterm-color.png, Action(Mouse 1) "Exec exec xterm")
*SimpleButtons: (1x1, Icon $[sbicons]/org.gnome.Nautilus.png, Colorset 10, Action(Mouse 1) "Exec exec dolphin")
*SimpleButtons: (1x1, Icon $[sbicons]/chromium.png, Colorset 10, Action(Mouse 1) "Exec exec chromium")
Click screenshot
Later I will change Transparent with RootTransparent… couldn’t get to work. I hope FVWM3 will include a better Transparent method.