Get notified when the user changes the theme (XP)?

Problem/Question/Abstract:

How to get notified when the user changes the theme (XP)?

Answer:

const
  WM_THEMECHANGED = $031A;

type
  TForm1 = class(TForm)
    {...}
  private
  public
    procedure WMTHEMECHANGED(var Msg: TMessage); message WM_THEMECHANGED;
  end;

  {...}

implementation

{...}

procedure TForm1.WMTHEMECHANGED(var Msg: TMessage);
begin
  Label1.Caption := 'Theme changed';
  Msg.Result := 0;
end;