Dear Mr. Hagen.

I apologize that I bother you once more, but I really need your help 
(deadline for my document is quite soon). :-(

I have to follow a rather complex document design. According to it 
tables must be framed with two different lines: The outer frame must be 
thick and the inner thin. Moreover, a part of the header (the first row) 
must be black. I didn't know how to do it, so I used Metapost in a 
company with the overlay mechanism provided with the ConTeXt.  It works 
well, but:

1) When a cell has black background, it turns its frame from black to 
white. It's awfull. How can I switch this behavior off?
2) The frame of the first and last row doesn't fit precisely to frames 
of the rest rows. What's wrong? Are those cell of a different sizes? How 
can I improve it?

Many thanks for your kind help.
Your sincerely
Michal Kvasnicka

P.S. I include a minimal (but not a small, I apologize) example.