Oh, okay... Yes, the N900 is fully themeable, using the same standard methods as desktop distros. However, there are a lot of UI elements that are specific to Maemo, so you can't just drop in the Ubuntu Human theme, say, and have it work (which is to say that it wouldn't look right, not necessarily that the theming engine would have any issues with it). There are plenty of different Maemo-specific themes being made, though, and some of them come with different icon sets. (The maemo.org theme, for example, comes with a more cartoony set of icons.) It's primarily a matter of time, and whether someone with the necessary skills decides it's worth their time. I've done some basic theming work on Ubuntu/GNOME before, and it didn't seem terribly hard in terms of technical knowledge needed, however, to do it "properly" is probably not as easy as one might think. If you're feeling like trying it out, take a poke at the files in /usr/share/themes and /usr/share/icons, for starters. If you have a desktop Linux install somewhere, you may want to wet your feet on that, as it's easier and faster to fiddle with themes on. I think there are some good tutorials, too.
EDIT: I forgot to mention... to my knowledge, Maemo doesn't handle icon themes any differently than other distros, so you should be able to adapt a given icon theme to Maemo without too much work (AFAIK, anyway).