HELSINKI, WITH ITS months-long stretches containing more darkness than light, is a natural fit for neon lighting. For businesses operating in the dark winter months, neon serves as a beacon that invites customers in from the cold. In contrast to other cities, where neon often takes on splashier forms, much of Helsinki’s neon is set in orderly sans-serifs, though here and there bursts of cursive and other stylized types still break through.