I am a web designer and have tested this cart for a year now.
If you have your own design there are no more than 10 interchangeable tags that integrate into your own style.
All you need is your index page as a template.
From that point on you simply take the default documents and copy and paste the PHP tags into the appropriate places in your own design and you are set to go.
The documentation (user manual) is one of the best I have ever encountered.
YOU DO have to know HTML and CSS.
YOU DO NOT have to know PHP (I do not or extremely limited).
They have a more than helpful forum.
I have noticed the people who have difficulties with this cart call themselves designers which means 9 times out of 10 they rely on MS Frontpage, Dreamweaver or some other WISIWIG form of software.
If you can not decipher a normal HTML document then it is doubtful you will be able to install any cart properly and it will be no different with this one.
My suggestion is try the demo ... it is free and once the cart is installed and integrated into your site entering product can be done by any novice. I have experimented on my business owner clients and had no problems. Again the cart admin is well documented.
Good luck and have fun trying the cart out :)