The file
The first two seasons opened with Bill Haley; from 1976 it was the Gimbel and Fox song, which went to the Top 5 on its own. Fox also wrote Killing Me Softly and the Love Boat theme.
The Fonz, the jukebox at Arnold's, and a theme that was itself a hit record. Older Gen X grew up on it; younger Gen X met it in reruns after school.
Sources: Wikipedia — Happy Days (song)