Hi Hans,
I love the style used for this manual. I tried to do the same thing, but it's a long way off !
I tried to find the source code to inspire me :-)
I did this :

\startuniqueMPgraphic{titled}
  path p ;
  p := unitsquare xscaled \overlaywidth yscaled \overlayheight ;
  pickup pencircle scaled 1mm ;
  linecap := squared ;
  drawoptions(withcolor \MPcolor{darkred}) ;
  draw ulcorner p -- urcorner p ;
  setbounds currentpicture to p ;
\stopuniqueMPgraphic

\defineoverlay[titled][\uniqueMPgraphic{titled}]

\startuseMPgraphic{triangledone}
fill lltriangle scaled 4cm withcolor darkred;
\stopuseMPgraphic

\defineoverlay[triangledone][\useMPgraphic{triangledone}]

\def\Triangledone#1%
  {\framed
      [background=triangledone,frame=off,offset=overlay]
  {\tfb\color[white]{\bf#1}}}

\startuseMPgraphic{triangledtwo}
fill lrtriangle scaled 4cm withcolor darkred;
\stopuseMPgraphic

\defineoverlay[triangledtwo][\useMPgraphic{triangledtwo}]

\def\Triangledtwo#1%
  {\framed
      [background=triangledtwo,frame=off,offset=overlay]
      {\tfb\color[white]{\bf#1}}} 

\setupheadertexts[margin][][\Triangledone\pagenumber][\Triangledtwo\pagenumber][]
  
\setupfootertexts[margin][]
                 [{\framed
                     [background=titled,frame=off,offset=8pt,align=left,
                      width=\dimexpr\leftmarginwidth-\leftedgewidth\relax]
                      {\darkred\getmarking[section]}}]

\setuppagenumbering[alternative=doublesided,location=]

\setuphead
   [section]
   [color=darkred]

\starttext

\startchapter[title={First Chapter}]
\startsection[title={First section}]
\dorecurse{5}{\input knuth}
\stoptext

Thank you
Fabrice