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;