To add a new skin or theme for Cubecart is such a painless process. Google for free skins if you don't want to pay. Download the file, which is normally zipped. Uncompress the file with WinRAR or a similar application.
Most skins come with a Readme text file, containing useful install and other info. Read it!
Fire up your FTP client and log in to your server. Drill down to the skins directory and copy across your new skin there, alongside the default install.
Log In to the admin account. Scroll down to the section headed "Styles & Misc". There's a drop down arrow from which you can select your preferred skin. Scroll to the bottom of the page and click "Update Settings" to save your changes.