I’m a CSC user and wanted to reply since I recently setup the cart on a custom design. If you know a bit about html and css you can probably do it yourself, but it might take some time to become familiar with the different smarty templates.
The current CSC skins use nested tables so it’s not ideal, but still possible to modify them to your design. I was able to do this by choosing a skin with a basic layout close to my design. I then turned off all the features I new I wouldn’t be using and changed the CSS colors to match my own. At this point you’ve got to dig into the smarty templates and start working in your images and layout code. I probably ended up editing about ten different files for this and was able to pick up a lot of great tips from this forum to tweak some cart features.
Overall i’m really happy with CSC. I’m in the process of creating a very basic css only skin, but there’s so many different smarty files it’s going to take awhile. If your design is pretty complex or requires special product layouts you might want to get a quote from CSC on customization options.