Hardly. I've been running a 5.0 hybrid for a while now, and there are many 5.0 themes, and many 4.5 themes that still work on 5.0.
Check out this forum for some great free ones:
BlackBerry Curve 83xx Themes - BlackBerry Forums at CrackBerry.com
Some of my faves from that site that work fine on 5.0.0.328:
SenseBerry Droid - Rain and Rock - FREE - BlackBerry Forums at CrackBerry.com
Blue Sky for BB Curve 83XX (4.5 OS) 5 options - BlackBerry Forums at CrackBerry.com