Customizing Maemo
This article will someday describe how everybody can customize Maemo5 to make it as personal as possible. There is a discussion concerning customization in http://talk.maemo.org/showthread.php?t=32845
Wallpapers
There is allready http://www.n900wallpapers.com/ page for wallpapers. Unfortunately you need a .desktop-file if you want to configure panoramic wallpapers. But situation seems to develop quickly. There is a conversation thread in http://talk.maemo.org/showthread.php?t=32427&highlight=n900wallpapers&page=12
Themes
Available themes can be installed straight from Application Manager. If you want to make your own theme, see below.
There is an application for making themes to Maemo OS2008. This will be updated as soon as possible to cover also N900 and Maemo5. See article Maemo Theme Maker HOWTO