PrestaShop comes with some default themes along with other 310 free features or even more. It is fine to keep the default theme and modules as long as you are trying to test the potential of your product. However it is recommended to change the theme instantly as there are hundreds of free themes available for download on Prestashop official Ecommerce site uploaded by individual theme developers. All you need to step up a new theme is few clicks.
Presentation Increase Sales:
Ecommerce success greatly depends on presentation. Every product or niche demands a different way of presentation. Like if you are setting up a cosmetic or cloth store than surely fancy theme will be better and if you are setting up office supplies store then surely you need a theme that looks mature and professional. The better you present high will be your sales.
Themes For Unique Identity:
Once you find your business have potential to grow than its time to spend a little more on your store. You may have uniqueness in products and services you are offering but what about appearance? Do you have a unique look? If not than better to have it now and let you store standout with respect to look, products and services. The uniqueness lead to a strong brand image and brand image leads to high sales.
People Buy Brands Not Products:
Recent studies have proved that people are more sensitive towards the brand image of the product they buy. They show strong affiliation with the brand and avoid using the non branded products. The best way develop strong brand image is to have unique identity.
For free and paid Prestashop themes – FME Modules
Prestashop Ecommerce Blog, Best Modules & Addons 