fransole,
To change the section material:
Place your custom wall or beam/footer.
Once placed, select it,
while it is still selected, open profile manger, choose edit profile,
then modify the fill settings in the little window showing the custom profile.
Now you can either re-save it as original name or choose new name for this profile.
To change the skin:
Select object and open the selection setting dialog , modify the skin material here. You have only one choice instead of the usual top, side, etc.
You might need to "rebuild" your view to see change.
Snap.