Hi Mikael,
I did not think about using Metapost, I tried with the keys of the command \framed without success.
I use the following code for my headings (thank you Aditya) and I would like to use your code instead.
Thank you.
Fabrice

\setupbackgrounds
    [header]
    [text]
    [frame=off,
     topframe=off,
     bottomframe=off,
     framecolor=,
     rulethickness=2pt]

  \defineframed
    [headerframed]
    [frame=off,
     height=fit,
     width=fit]

  \definecolor[ColorA][0.8(white)]
 

  \startsetups [headertext]

    \setupheadertexts
      [{\startframed [headerframed]
      [foregroundstyle=bold,background=color,backgroundcolor=ColorA]
      \getvariable{headertext}{title} n\high{o}\,\getvariable{headertext}{number}
      \stopframed}]