I had taken the following background taken from the Green presentation style (s-pre-02), working satisfactorily without need to define the OverlayLineWidth parameter:

\startuniqueMPgraphic{PageBackground}

  fill unitsquare

    xyscaled(OverlayWidth,OverlayHeight)

    withcolor OverlayColor ;

  draw unitsquare

    xyscaled(OverlayWidth,OverlayHeight)

    enlarged (-2*OverlayLineWidth)

    withpen pencircle scaled OverlayLineWidth

    withcolor OverlayLineColor ;

\stopuniqueMPgraphic

Now suddenly the OverlayLineWidth seems to have turned into zero and the frames do not come out as previously. What happened? Did I miss some change or need to explict initialization? Where?