def change_column_font(column_index, change_type, arg, font, xf)
      validate_workbook
      ensure_cell_exists(0, column_index)

      xf = workbook.register_new_font(font, xf)
      cols.get_range(column_index).style_index = workbook.register_new_xf(xf)

      sheet_data.rows.each { |row|
        c = row && row[column_index]
        c.font_switch(change_type, arg) unless c.nil?
      }
    end
def change_column_font(column_index, change_type, arg, font, xf)
      validate_workbook
      ensure_cell_exists(0, column_index)

      xf = workbook.register_new_font(font, xf)
      cols.get_range(column_index).style_index = workbook.register_new_xf(xf)

      sheet_data.rows.each { |row|
        c = row && row[column_index]
        c.font_switch(change_type, arg) unless c.nil?
      }
    end
Helper method to update the fonts and cell styles array
 main method to change font, called from each separate font mutator method