The present dress code on Seabourn is one Formal Optional night per seven day cruise. So, for example, on my recent 9 day cruise on the Seabourn Odyssey there was 1 Formal Optional night, 6 Elegant Casual and 2 Casual nights.
Formal Optional is where you can dress formally (tux/dark suit and gowns) if you like, but the rest of the ship is requested to dress at least Elegant Casual. What does that mean? It means a jacket, but no tie for men and a dress, skirt or pants outfit for women. However, what you will find is that almost all men will wear a tie Formal Optional evenings.
BTW, casual is not NCL casual. It means a collared shirt for men and if you dine in The Restaurant you do not need a jacket. Jeans are not permitted...even $300 a pair jeans. (I mean who looks at price tags when what your fellow diner's see is denim?)
On my last cruise I did not see any issues to speak of. Seabourn does have some "emergency" jackets for men who do not bring them to The Restaurant and do not wish to return to their suites to retreive on.
That is, however, only The Restaurant. If you dine in The Colonnade or Restaurant 2 or the Patio Grill, no jackets are required.