Использование set_format метод:
require 'spreadsheet'
book = Spreadsheet::Workbook.new
sheet1 = book.create_worksheet
format = Spreadsheet::Format.new :color => :blue,
:weight => :bold,
:size => 18
row = sheet1.row(0)
row[0] = 'test0'
row[1] = 'test1'
row.set_format(0, format) # set format for the first cell
book.write 'C:\\test.xls'