Good afternoon, I want to create a theme for the nokia n9 and I do not know where to take it or as technical information. You can download a pdf or a book about it, or web page link. I thank you for the answer.

how to modifique file css into theme folder?

Here you will find some informations. I think it is a good start.
Maybe someone knows some better source for reading.

Explanations:
http://leimu.drupalgardens.com/content/themechanger

Themechanger:
https://openrepos.net/content/rzr/themechanger

Source code:
https://github.com/matrixx/ThemeChanger

Theme settings:
http://vesuri.jormas.com/themesettings/