def styles
      @styles ||= self.workbook.styles
    end
shortcut method to access styles direclty from the worksheet
This lets us do stuff like:
@example
p = Axlsx::Package.new
p.workbook.add_worksheet(:name => 'foo') do |sheet|
my_style = sheet.styles.add_style { :bg_color => "FF0000" }
sheet.add_row ['Oh No!'], :styles => my_style
end
p.serialize 'foo.xlsx'