Comparing to Custom Magento themes and templates all other themes and templates will be lightweight, W3C XHML & CSS validation and SEO compliant. Starting with custom theme design and it implemented for a Magento websites. Also magento is the best resource for E Commerce. Making the rights to use the design, whether it is an existing HTML design for your site, an existing CMS theme design, even a jpeg image, then develop that design into a full Magento theme. With optimized mark up website for the best SEO practice to market place.