Winter is coming. With the darkest day of the year — Dec. 21 — just around the corner, right about now you might be dreaming of swapping seasonal blahs for Caribbean blue. But while fun in the sun can be a welcome interruption during the colder months, there’s an art to the mood-boosting getaway — beyond merely jetting off to the tropics for a dose of Vitamin D that doesn’t come in capsule form, a behavioral expert suggests. Rather than simply…