Update: Google Chrome now officially supports themes, you can read more about installing and using official Google Chrome themes.
We had done a post earlier about how to install themes in Google Chrome, but things change so soon and we already have a unofficial Google Theme manager called XChrome. XChrome takes the hard part out of installing themes.
So unlike earlier, you do not have to copy and replace the default.dll files, XChrome Theme Manager will easily do it for you.
XChrome theme manager is definitely useful since it automates the process of installing new themes, and maintains a history of themes you have installed for Google Chrome, The software is still in beta stages so you can expect bugs in it. In addition to installing themes you can also edit them and create your own Google Chrome theme with the included Theme maker.
Download XChrome Theme Manager [Link removed due to Malware alerts]