How to draw the element part in right to left (RTL) direction when drawing with DrawThemeBackground?

I'm trying to draw the ttGlyphClosed element of Explorer::Treeview class in right to left direction (like when the BiDiMode would be bdLeftToRight). I have a problem, that I don't know how to make my offscreen bitmap to be transparent. The bitmap's background is always white.

I'm using the following code to mirror the image:

procedure TForm5.FormPaint(Sender: TObject);
var
  bm: TBitmap;
  ARect: TRect;
  Details: TThemedElementDetails;
begin    
  if ExplorerTreeviewhTheme = 0 then
    ExplorerTreeviewhTheme := OpenThemeData(0, 'Explorer::Treeview');

  ARect := Rect(20, 20, 40, 40);
  Details := ThemeServices.GetElementDetails(ttGlyphClosed);
  DrawThemeBackground(ExplorerTreeviewhTheme, Canvas.Handle,
    Details.Part, Details.State, ARect, nil); //Ok

  bm := TBitmap.Create;
  try
    bm.Width := 20;
    bm.Height := 20;

    ARect := Rect(00, 00, 20, 20);
    DrawThemeBackground(ExplorerTreeviewhTheme, bm.Canvas.Handle,
      Details.Part, Details.State, ARect, nil);

    // rendered result has white background
    Canvas.Draw(60, 10, bm);    
    // rendered result is mirrored but has also white background
    StretchBlt(Canvas.Handle, 100, 10, -20, 20, bm.Canvas.Handle, 0, 0, 20, 20, SRCCOPY);
  finally
    bm.Free;
  end;    
end;

The question is how to mirror element drawn by the DrawThemeBackground function (for RTL reading) or how to use this function for RTL (right to left) rendering ?

Use SetLayout as TLama showed in his now deleted answer to switch the layout of the canvas before you draw.

function SetLayout(hdc: HDC; dwLayout: DWORD): DWORD; stdcall;
  external 'gdi32' name 'SetLayout';

const
  LAYOUT_RTL = $00000001;

procedure TForm1.FormPaint(Sender: TObject);
var
  ExplorerTreeviewhTheme: HTHEME;
  Details: TThemedElementDetails;
  ARect: TRect;
  Size: TSize;
begin
  ExplorerTreeviewhTheme := OpenThemeData(Handle, 'Explorer::Treeview');
  Details := ThemeServices.GetElementDetails(ttGlyphClosed);
  GetThemePartSize(ExplorerTreeviewhTheme, Canvas.Handle, Details.Part,
      Details.State, nil, TS_DRAW, Size);

  ARect := Rect(20, 30, 20 + Size.cx, 30 + Size.cy);
  
  // normal layout
  DrawThemeBackground(ExplorerTreeviewhTheme, Canvas.Handle,
                      Details.Part, Details.State, ARect, nil);

  // switched layout
  SetLayout(Canvas.Handle, LAYOUT_RTL);
  
  // calculate the rectangle for RTL as if it's in LTR
  OffsetRect(ARect, 0, Size.cy); // align to the bottom of the first image so that we can see
  ARect.Left := ClientWidth - ARect.Left - Size.cx;
  ARect.Right := ARect.Left + Size.cx;

  DrawThemeBackground(ExplorerTreeviewhTheme, Canvas.Handle,
                      Details.Part, Details.State, ARect, nil);
  
  // restore layout
  SetLayout(Canvas.Handle, 0);
  CloseThemeData(ExplorerTreeviewhTheme);
end;

Output:

The theme api is drawing a 6px wide triangle in a 16px part size (W7-aero). Since you won't be able to know the placement of the image in the part, you won't be able to align it any better.