This icon is the best icon. It's also very indicative, at the very least, of how one can expect Thomas to dress at formal vampire functions. In canon, the reason he was wearing a white loincloth was because he was trying to represent that he was a vampire of the White Court, you see. The wings are because it was a costume party. The lack of anything else is because Thomas doesn't usually wear shirts for official vampire business. Except once. It was a Buffy shirt.