After the recent firmware update, I've found a weird bug. Actually, after every boot, no matter what theme I've selected before, my N900 switches back to the default theme, the one called Nokia Nseries. The wallpaper remains the one of the previous theme.
I wonder if this is actually happening to some other people, I've reported that to Bugzilla, if you are experiencing this have a look there.

https://bugs.maemo.org/show_bug.cgi?id=7852

If you have already done that update, please try changing the theme and rebooting.