Although the debate about whether the music for Theme's Time closing credits was the theme from Top Cat or not has largely abated, I've always wondered where that jazzy acoustic piano version came from.
Thanks to a purchase through the Dreamtime store, now I know. The song was probably taken from a 1996 CD, later re-issued in 2002, and available in the U.S. as an import at Amazon, Tunes from The Toons: The Best of Hanna-Barbera. The CD is also available at Amazon U.K. and Amazon.de.
If you follow the link(s) and scroll down a bit to Track #4, which is listed as Top Cat (Underscore) on the U.S. page, and play the clip, you'll find that it is indeed the music from TTRH's closing theme.