Unlike other open source product like Joomla, drupal, mambo etc. that separate the presentation layer out from the core program codes, which allow an easier implementation of website theme customization. osCommerce design is mixed into the core program codes, customizing website design is basically looking into HTML styling codes in between core program codes.
There are some oscommerce theme customization available if you are interested in changing the look and feel of your oscommerce website, each piece of code may have different implementation steps, you will have to check the README file for the detail instructions on getting the customized code in your oscommerce website.
oscommerce contributed modules
Similar to installing new oscommerce templates, getting a contributed oscommerce addons requires different setup process, some requires copying additional files/folders to the oscommerce system and some require modification to the core oscommerce codes, you will have to download the module zip file, uncompress it and check the documentations included in the zip file for details on getting the modules to work.