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You can also choose to create your own theme, or have it created for you by a developer. Refer to the online Designer Guide for help, at http://doc.prestashop.com/
Once your theme is installed, you should explore it fully in order to know it by heart and be able to help customers out of a situation.
You can and should customize some of the theme's aspects, most notably its logo in various situations (header, mail, invoice). This is done using the "Themes" preferences page, which you can find under the "Preferences" tab.
Choose your modules
PrestaShop comes bundled with more than a hundred modules. These are very varied: analytics, front-office features, payment, shipping... You should explore the available modules in full, in order to know which ones you might want to enable, and which you'd rather keep disabled.
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Every time you activate and configure a module, make sure that it does work will well within the confines of your theme, if its features impact your shop's front-end.
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