Wow. Here I thought this was a dumb question and that somebody would have an obvious answer. But these are quite diversified. A Google search also reveals an assortment of theories, but no real answers. I wonder if WB intended that.
The Warner Bros. cartoon title cards always reminded me of two other art deco classics:
The proscemium at Radio City Musical Hall, and
Maybe they were the inspirations for the title cards.
Elwood P. Dowd.
Harvey was not a rabbit. He was a pooka.