To change the theme for Google Chrome you will need to replace the default.dll file with the new theme file. Location for theme folder are as follows Replace C with your Windows directory, UserName with the user name you have installed chromed with, the number is the version for Google Chrome.

Make sure to keep a backup of the default.dll, before you overwrite it with a new one. DeviantArt already has a Picthblack theme which you can use to replace the default Google Chrome theme.