It's simple. All you need to do is replace the default.dll file with the new theme file. Location for theme folder are as follows:

For Vista Users: C:\Users\UserName\AppData\Local\Google\Chrome\Application\0.2.149.27\Themes\

For XP Users: C:\Documents and Settings\UserName\Local Settings\Application Data\Google\Chrome\Application\0.2.149.27\Themes\

Google I think doesnt allow to change the theme by default so what your goign to do is to download google chrome theme. After you have done it is to to extract the “default.dll” file.

For Windows XP users you need to extract the file ;

Extract it to : C:\Documents and Settings\User\Local Settings\Application Data\Google\Chrome\Application.2.149.27\Themes\

For Windows Vista users

Extract it to : C:\Users\UserName\AppData\Local\Google\Chrome\Application.2.149.27\Themes\

And then you need to restart your google chrome to see the new theme.