Fortunately, all Unity decorations are defined by the Unity gtk3 theme (see this document), and one can remove the top decoration with one little change.
However, just to be on the safe side, I do make a backup copy of a whole theme. Here is what I do:
sudo cp -a Radiance Foopance
sudo cp index.theme index.theme.orig
sudo sh -c "sed 's/Radiance/Foopance/g' index.theme.orig > index.theme"
One last thing to do is to modify (as root) the file
sudo gedit /usr/share/themes/Foopance/gtk-3.0/apps/unity.css
Change the line
-UnityDecoration-extents: 28px 0 0 0; to
-UnityDecoration-extents: 0 0 0 0;
This is all it takes. Now you can select the "Foopance" theme using the Unity Tweak Tool. The window decorations will no longer be shown!