To download the theme file, hover over and click a thumbnail.
You will not be able to replace the file if you are using Chrome so you
will have to download it to a separate directory and copy it over.

The file needs to stay named "default.dll" and placed in this directory:

Windows XP Users:
C:\Documents and Settings\[User Name]\Local Settings\
Application Data\Google\Chrome\Application\[Version]\Themes\

Windows Vista Users:
C:\Users\[User Name]\AppData\Local\Google\Chrome\Application\[Version]\Themes\

Make sure you create a backup of your original "default.dll"
file before overwriting it with the new one.

If you're having trouble finding the directory you can also right click on the shortcut to the program and choose properties then press the
"Open File Location" button.