Turn your system on, open the web browser and search for Google Talk/GTalk themes or skins for the Windows you use.
Execute .exe file to install or extract the zipped wp-content/uploads and copy the theme on the path mentioned for the operating system you use.
For Windows Vista
C:\Documents and Settings\<user name>\Local Settings\Application Data\Google\Google Talk\themes\user\chat\
For Windows XP
C:\Documents and Settings\<user name>\Local Settings\Application Data\Google\Google Talk\themes\system\chat
For Windows 7
C:\Users\<user name>\AppData\Local\Google\Google Talk\themes\system\chat
Launch Google Talk, login it and click on the 'settings' tab.
Select Appearance where the list of chat themes is shown with preview.
Select the required theme/skin and click OK to confirm your selection.