How do I create an ecommerce website with multiple theme options for users?

Hello guys... I'm in a dilemma. I want to create a website(ecommerce) where users can change themes. I would love to create my own set of themes with cool colours but I dont know how to go about it easily. I need good graphics for my website(very good looking that's why I prefer to do it myself) but dont know if there's a tool to do that or what skills (aside css) I need to learn. On a second thought would be better I go for an already made script with themes? If I need to go for an already made script, which would you suggest?

I intend to work with php