installing htc theme - require installed ROM?

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I have to admit, I'm not a techie. At all. Whenever I hear/read "ROM UPDATE!" or "ROOTING YOUR PHONE!", I totally draw blank and I have no clue what that means.
With that said, I was wondering how you install themes onto your HTC Sapphire/Magic and if it requires your phone to be rooted .. or under some sort of rom?

I know what you mean, ive had wm phones in the past. Took me a bit to understand what "flash" meant lol. I've been reading, http://www.ryebrye.com/blog/2009/08/...k-in-progress/
Although, (not to be stealing any fire from rooting new rom) you can download aHome from the market and they have hero themes you can install for them. Requires no "rooting" at all.