When winter winds howl through their Illinois town, Dan and Karen Heller think of Summer — Chicago designer Summer Thornton, that is. The smashing good looks of their getaway north of Naples, Florida, as bright and breezy as a perfect July day, are thanks entirely to Thornton living up to her name. The couple purposely chose a house in a community that offers golf, tennis, and all manner of water sports just steps from their front door. Their hope was to lure their two married sons, three young grandchildren, brothers of daughters-in-law, Dan's 80-something parents, assorted friends — in short, everyone they love — to come enjoy it with them. The place needed to be a happy fit for multiple generations. "Is it possible to be casual but also elegant?" was the question Karen posed at the start. That pretty much describes Thornton's style to a T, so she dove right in with her typical brio. "I do equate happiness with color," she confesses, but says her complex palettes always start with a surprisingly simple recipe: "First I ask myself, What is the main white? Should it be a cool white? Or a cloudy white?" Here, she chose paint in an alabaster shade to warm the great room, which has 20-foot ceilings, then matched the fabrics to that creamy color. "Next, I ask myself, What will be our main contrast color? Here it's indigo, but in another house, it might be raisin or even black," she says. "Strong contrast creates bold crispness. I don't like muddiness. The object is to keep things cheery!"To underscore the blue-and-white theme, Thornton helped hunt down hundreds of pieces of Chinese export-style porcelain that she and Karen then filled with flowers, layered on consoles, and even enlisted as end tables. Coral arrived by the crateful, too, and was scattered extravagantly. "A house has more impact if you go all the way with accessories," Thornton says. Besides, the collections make the place feel settled despite having been built just two years ago.