DENVER (Reuters) - Democratic presidential candidate Barack Obama's big speech on Thursday night will be delivered from an elaborate columned stage resembling a miniature Greek temple. The stage, similar to structures used for rock concerts, has been set up at the 50-yard-line, the midpoint of Invesco Field, the stadium where the Denver Broncos' National Football League team plays. Some 80,000 supporters will see Obama appear from between plywood columns painted off-white, reminiscent of Washington's Capitol building or even the White House, to accept the party's nomination for president. He will stride out to a raised platform to a podium that...