Is Ashton Kutcher singing theme song?

You're likely thinking of the theme song for the popular sitcom "That '70s Show," which Ashton Kutcher starred in.

The theme song is not sung by Ashton Kutcher. It is a popular instrumental piece called "In the Flesh?" by the rock band Pink Floyd.

While Ashton Kutcher doesn't sing the theme song, he does appear in the opening credits alongside the rest of the cast.