After some more testing, it seems to come from the Ubuntu Theme "Ambiance".
If I choose another theme, like "Radiance", the contrast is ok and menus are readable.
I tested the same sweethome3d version on Ubuntu Lucid 10.04.4 : the contrast is ok for both themes Ambiance and Radiance.
So this seems to be specific to Precise (and maybe Maverick/Natty/Oneiric : I did not test)
After some more testing, it seems to come from the Ubuntu Theme "Ambiance".
If I choose another theme, like "Radiance", the contrast is ok and menus are readable.
I tested the same sweethome3d version on Ubuntu Lucid 10.04.4 : the contrast is ok for both themes Ambiance and Radiance. Natty/Oneiric : I did not test)
So this seems to be specific to Precise (and maybe Maverick/