All together now, "Some walk by niiiiight, Some fly by daaaaay, Nothing could change yoooou, Set and sure of the waaaaay."
If you don't know the theme tune, you're probably in your 20s. It's from 80s TV show Moonlighting, Bruce Willis' first proper foray into the public eye. He starred opposite Cybil Shepherd as a pair of private eye's who quibbled more than your average married couple, but they had this amazing sexual chemistry - thanks to not being a married couple.
Anyway, Bruce's stint on the show resulted in an appearance on Letterman back in the day. Obviously, it's a time Bruce recalls fondly. So fondly, that he saw fit to duplicate the look and, man, it's a struggle to him and Future Bruce apart.
For those desperately seeking the full theme tune to Moonlighting, here you are. You're welcome.