I finally created iGoogle theme by my own!!
I thought it was very difficult to make xml file, but it was not. Here is the instruction : http://code.google.com/intl/en/apis/themes/docs/dev_guide.html
The header image, logo, text colors, and more can all change throughout the day based on time of day, which is called "a dynamic theme". I didn't set it up this time, but I will!!