As per Viking:
What is the dress code?
During the day, dress is casual including shorts (if the season is warm), slacks or jeans and comfortable shoes for walking tours. Swimsuits, brief shorts, cover-ups and exercise attire should be reserved for the Fitness Center, pool areas and Sports Deck. There are no “formal nights” in the evening; evening dress is “elegant casual” for all dining venues, performances and special events. On these occasions, required attire for ladies includes a dress, skirt or slacks with a sweater or blouse; for gentlemen, trousers and a collared shirt. A tie and jacket are optional; jeans are not permitted. The evening dress excludes World Café where the dress remains casual after 6:00 PM.
Some shore excursions visit religious sites that require modesty of dress. To avoid being denied entry, we recommend men wear long pants and refrain from sleeveless shirts; and for women, clothing that covers the knees and tops that are not sleeveless or too revealing.