Mike,
Although the majority of the men will be in dark suits or tuxedos, as long as you are wearing a suit and tie you will be fine and should feel quite comfortable eating in the main dining room.
For tips about embarking from Barcelona, you might want to check out the information we have on PortReviews.
http://europeportreviews.com/BarcelonaSpain.htm
The Brilliance is a beautiful ship - you are going to love her!!!
Nancy