startuseMPgraphic{mp:tcolorbox}
  path p, q; numeric w, h, o;
  w := OverlayWidth; h := OverlayHeight; o := BodyFontSize;
  q := (0,0) -- (0,h) -- (w,h) -- (w,0) -- cycle;
  p := (0,0) -- (0,h+2o) -- (w,h+2o) -- (w,0) -- cycle;
  p := p cornered (o);
  fill p withcolor OverlayColor;
  draw p withcolor OverlayLineColor withpen pencircle scaled OverlayLineWidth;
  p := (0,h) -- (0,h+2o) -- (w,h+2o) -- (w,h) -- cycle;
  p := p cornered (o);
  filldraw p -- cycle withcolor OverlayLineColor;
  draw textext.rt(MPstring{tcolorbox}) shifted (o,h+o) withcolor white;
  setbounds currentpicture to q;
stopuseMPgraphic

defineoverlay[tcolorbox][useMPgraphic{mp:tcolorbox}]

definesetframetitle
  {setMPtext{tcolorbox}{strutframedparameter{title}}}

defineframedtext
  [tcolorbox]
  [frame=off,
   background=tcolorbox,
   backgroundcolor=white,
   framecolor=darkred,
   rulethickness=2pt,
   extras=setframetitle]

starttext

starttcolorbox
  [title={Knuth},
   backgroundcolor=lightgray]

  samplefile{knuth}

stoptcolorbox

stoptext