Theme for the western world: cool and casual covering. Witness the shirt-happy gentlemen on the starboard page (each, admittedly, just a shell of his former self). Packed with sea-fresh departures in cut, collars and colors, the sport shirt shenanigans of the international fashion set are exciting to see. Note the collar on the blue Italian job: a short-point widespread with just a touch of roll, opening into a deep, deep neck. Note also the colorful clutch of stripes (heretofore so dashing on tigers, hot-blooded Sioux and barber poles) blended with a subtle, almost languid, effect. Whether you take your Aprils in Pomona or Princeton, chances are your favorite sport shirts this season were first seen in Paris or Portofino. Going at it clockwise, starting with the long-sleeved guy lunging for the Collins cooler, we have, voilà, a brown and black striped affair in a ribbon weave of Egyptian cotton, with a Continental mitered collar, washable, $15. Next, a Copen blue Italian shirt of pure silk with half sleeves, horizontal stripes of red and black, Italian collar, non-washable, $18.50. Following, a French boatneck pullover fashioned of spun cotton with the Riviera sleeve, smoothly blended horizontal stripes of gold, russet and blue, washable, $5.95. Then, a classic British boots-and-saddles pattern of Egyptian maize-colored cotton, trimmed in hunter's green and black, with half sleeves, washable, $12.95. Finally, the bicyclists' bonanza: a bright crimson shirt of Acrilan with racers' collar piped in black and white, washable, $10.95.