This article at Fast Company about top-down humor in the workplace asks an intriguing question about work culture; "When was the last time you teased your boss?"
I was at an event in the Midwest a few years back listening to war stories about partners at law firms. At one firm years ago (so the story goes) a particularly successful partner was known to leave notes taped to associates chairs stating "You're Fired" if the associate arrived at the office late or left early. At another firm, a particular partner would fire any receptionist that put him on hold when he called in. OOOF!
The company referenced in the Fast Company article is at one end of the spectrum... the stories above, if near true, would put those firms at the other end.
I know from my own experience that working with people that have and show humor is a tremendous boost within the culture. One of the questions posed recently in preparation for Halloween at the office was, "will so-and-so partner be wearing the Hawaiian cococut costume?" (One of the floors had a tropical theme). On another floor the theme was "inappropriate workplace clothing" and a partner came dressed in the most tacky, ill-fitting yellow leisure suit and glued on mutton chops to round out his "look". AND, he saw clients that day!!
I like knowing that in a business that is very serious by the nature of the work we do my firm at least knows how to laugh with and around each other. This kind of culture can only work from top down. I hope that your firms enjoy a bit of this motivational freedom.