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."

Nexus One now has multitouch capabilities

The latest update on Google Nexus One are:
- user can use multitouch capabilities
- Google Goggles: this mobile application will now be available directly on your device by launching it from your All Apps menu. Just use your Nexus One camera to start searching the web
- Google Maps: the Maps application with be updated to a new version, Google Maps 3.4
-3G connectivity: we will provide a general fix to help improve 3G connectivity on some Nexus One phones