Use Hidden Themes and Download Themes
You don’t have to select a default theme. The Desktop Background and Window Color options enable you to customize your desktop as much as you like. Everything from the wallpaper to the color intensity of the windows and menu bars.
If you have the Windows 7 RC version there are hidden themes that can be easily enabled to allow a few more display options. To enable them you will need to have the option to Show Hidden Folders enabled. To do this, open Explorer, select Organize in the top left, Folder Options, the View tab, then select Show hidden files, folders, and drives.
Next, navigate to C:\Windows|Globalization\MCT, assuming Windows is on your C: drive. Here you will see a list of folders beginning MCT. These are the folders that contain region specific information. Double clicking on one will display a Theme folder, double clicking on that will present a Windows Theme File for the region. Depending on what version of Windows 7 you are using, these options may differ. For example, MCT-AU is for Australia, the Theme file contains a Windows Theme File with Australian related desktop themes. Double clicking the theme file will install it for you to use, whatever your region, and you can do this for all the MCT region files you have. They are mainly variations on a theme but do contain some good wallpapers.
If you have the Windows 7 RTM or retail version you can follow this link to download these themes to your machine. Choose to open with the themepackfile to automatically add them to your theme choice menu.