{
 When the application displays a Help Hint,
 it creates an instance of HintWindowClass to represent
 the window used for displaying the hint.
 Applications can customize this window by creating a
 descendant of THintWindow and assigning it to the
 HintWindowClass variable at application startup.
}
type
 TMyHintWindow = class(THintWindow)
  constructor Create(AOwner: TComponent); override;
 end;

implementation
{....}
constructor TMyHintWindow.Create(AOwner: TComponent);
begin
 inherited Create(AOwner);
 with Canvas.Font do
 begin
  Name := 'Arial';
  Size := Size + 5;
  Style := [fsBold];
 end;
end;
procedure TForm2.FormCreate(Sender: TObject);
begin
 HintWindowClass := TMyHintWindow;
 Application.ShowHint := False;
 Application.ShowHint := True;
end;

type

 TForm1 = class(TForm)

 procedure FormCreate(Sender: TObject);

 private

  {Private declarations}

 public

  procedure MyShowHint(var HintStr: string; var CanShow: Boolean;var HintInfo: THintInfo);

  {Public declarations}

end;

var

 Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.MyShowHint(var HintStr: string; var CanShow: Boolean; var HintInfo: THintInfo);

var

 i: integer;

begin

 for i := 0 to Application.ComponentCount - 1 do

  if Application.Components[i] is THintWindow then

  with THintWindow(Application.Components[i]).Canvas do

  begin

  Font.name := 'Arial';

  Font.Size := 18;

  Font.Style := [fsBold];

  HintInfo.HintColor := clWhite;

  end;

end;

procedure TForm1.FormCreate(Sender: TObject);

begin

 Application.OnShowHint := MyShowHint;

end;