I like Ubuntu Linux, but I'm not sure I like the brown/orange desktop theme. Here is how I changed the desktop theme on my Mini 9. I was looking for ways to customize it and ran across a way to make it look like a Mac:
Not every step in these instructions worked though. For example, the Dock, Elegant Glass feature, some fonts and the Global Menu didn't work. I didn't try the splash screens or screenlets. Don't try this unless you feel comfortable restoring your system from the recovery CD. Changing your desktop theme to look like a Mac probably isn't supported by Dell either. That said, I didn't have to restore my system. I recommend trying this on an old computer first before you try changing a new Mini 9. By the time I downloaded everything and several retries, it used about 65MB of disk space.
I only had 2 problems. First, I couldn't get the Global Menu to work. Now I have an extra File Menu bar on some windows. It doesn't really bother me, but it does take up real estate on my small 8.9 inch screen. My other complaint is that some windows have funny colors sometimes. If you close the window and open it again the window usually changes back to the right colors. I can live with it.
I haven't tried changing back to the standard desktop theme yet. I guess you could. I like the Mac look and I'll probably keep using it. Ok, it's not a real Mac and it's not really running OS X. It's still pretty cool though. A $249 Mini 9n running Ubuntu Linux that looks like a Mac is way cheaper than a real $1799 Macbook Air.