It really doesn't happen that often - a TV theme hitting the charts and soaring into the Top 10. Especially these days, when the TV theme song is all but extinct. But back in the 60s and 70s, theme songs were all the rage. And sometimes, they became almost as big a hit as the shows they introduced. 

Here's the story of how Pratt & McLain came to put the theme to "Happy Days" on disc. (And remember, the original theme song was Bill Haley's Rock Around the Clock.) It happened because listeners began demanding it. 

Constant radio requests led to a Happy Days theme song record in mere days