private
    { Private declarations } 
    FullRgn, ClientRgn, CtlRgn: THandle;
    procedure MakeTransparent;
    procedure UndoTransparent;

{...} 

implementation

{...} 

procedure TForm1.MakeTransparent;
var
  AControl: TControl;
  A, Margin, X, Y, CtlX, CtlY: Integer;
begin
  Margin := (Width - ClientWidth) div 2;
  FullRgn := CreateRectRgn(0, 0, Width, Height);
  X := Margin;
  Y := Height - ClientHeight - Margin;
  ClientRgn := CreateRectRgn(X, Y, X + ClientWidth, Y + ClientHeight);
  CombineRgn(FullRgn, FullRgn, ClientRgn, RGN_DIFF);
  for A := 0 to ControlCount - 1 do
  begin
    AControl := Controls[A];
    if (AControl is TWinControl) or (AControl is TGraphicControl) then with AControl do
      begin
        if Visible then
        begin
          CtlX := X + Left;
          CtlY := Y + Top;
          CtlRgn := CreateRectRgn(CtlX, CtlY, CtlX + Width, CtlY + Height);
          CombineRgn(FullRgn, FullRgn, CtlRgn, RGN_OR);
        end;
      end;
  end;
  SetWindowRgn(Handle, FullRgn, True);
end;

procedure TForm1.UndoTransparent;
begin
  FullRgn := CreateRectRgn(0, 0, Width, Height);
  CombineRgn(FullRgn, FullRgn, FullRgn, RGN_COPY);
  SetWindowRgn(Handle, FullRgn, True);
end;