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;