The Knoppix/Debian choice in knoppix-installer is not about the cosmetics of KDE. The Knoppix choice keeps all the stuff from the CD (hardware autodetection on each boot, for example) , and it's the one you want if you're going to remaster the CD. The Debian choice gets Knoppix behaving more like a "normal" hard-drive installation that doesn't go through a complete reconfiguration routine each time.

Ok, but why does root's KDE look different from user's KDE? Mainly to provide strong enough visual cues to keep you from forgetting that you're root and doing something reckless. Some other distributions go much further with this than Knoppix. The last time I looked at Mandrake, root's KDE desktop was blood red, and pure Debian acts shocked that root would ever consider using X at all since all of its true magic is best and most safely performed at the command line.

It's easy to get the look and feel of user's KDE to look like root's KDE -- fiddle with the settings in KDE Control Center.