What is the dark knight theme song one that they keep playing on commercials?

The theme song you're thinking of is called "Why So Serious?" It's the main theme from the film The Dark Knight, composed by Hans Zimmer.

While it's not the *entire* theme, the specific section used in commercials is the iconic, dissonant, and slightly eerie melody that often plays when the Joker is on screen.

It's a powerful piece of music that perfectly captures the film's dark and intense tone.