The only reference I can find in the "Help" files is using the "HTML Editor" to change size and color of borders. I had assumed something of that nature would be shown in the "Properties" panel of the object.
Sorry, I think to have made a wrong thing on the help. Please act on the table object using the "Cell Properties" button; the "Table Cell Properties" Window will open (see it on the help under the "dialogs" section)
I see now - you use table properties first to put the table background color and use cellspacing for width of border. Then you color the cells. I assume Davide did it this way due to wanting the program to have code that validates properly.