Once you download the custom Google Chrome themes from here , you can start to change the default themes.
To change the theme for Google Chrome you will need to replace the default.dll file with the new theme file. Locations for theme folder are as follows:
For Vista Users: C:\Users\AppData\Local\Google\Chrome\User Data,
For XP Users: C:\Documents and Settings\Local Settings\Application Data\Google\Chrome\User Data and go to /Application/Chrome version/Themes folder. The “Chrome version” folder will be in the form of 0.2.149.29 (The number will vary according to the version of Chrome that you installed). Replace the “default.dll” file in the folder with the “default.dll” file that you have just downloaded .
Don’t forgot to backup the Google Chrome default.dll first before you overwrite it with custom Chrome theme.