How customizable are Android OS phones in terms of display/appearance?

How customizable are Android OS phones in terms of display/appearance?
(regarding fonts, icons, tabs, menu, bars, colors, etc?)
Also, how are they if compared with Android OS phones W/ HTC Sense?
Does this adds more customization, or does it limit?

I may be comparing HTC Desire with Nexus One.
I like the top bar icons/notifications/battery display of Nexus One, can that be retained on HTC Desire?


"The HTC Desire & the HTC Nexus One run the Android 2.1 OS, and share the same specs (although I think the Desire's screen is slightly larger). HTC's senseUI is simply laid on top of the stock Android 2.1 OS. The notification bar is retained, however the notification bar on the Desire is black because it runs the senseUI. The Desire has not been rooted yet, like the Nexus."