uses
  FMX.Header;

procedure TForm1.StringGrid1ApplyStyleLookup(Sender: TObject);
var
  Header: THeader;
  i: Integer;
begin
  Header := THeader((Sender as TStringGrid).FindStyleResource('header'));
  if Assigned(Header) then
  begin
    for i := 0 to Header.Count-1 do
      with Header.Items[i].Font do
      begin
        Size := 32;
        Style := [TFontStyle.fsBold];
      end;
    Header.Height := 48;
  end;
  TStringGrid(Sender).RealignContent;
end;