how have you resolved?
I apologize if I did not answer.
I solved the problem bypassing.
I created 2 sets of icons, for the two themes of AutoCAD.
I maintained then 2 *.BMP file for each icon, I have not done the DLL
So according to the variable COLORTHEME I decided to copy (via code lisp) files *.BMP correct.
It is not a very clean solution but it works.