EShop supports multiple themes so developers can add more themes to the shop. You can check the documentation of Customize Layouts to customize layouts of EShop or develop your own theme for EShop if you want.
Go to EShop -> Plugins -> Themes to access to themes section of EShop. EShop team developed some themes for EShop, you can find them at: http://joomdonation.com/joomla-templates/eshop-templates.html
If you have a theme package, then in the themes list page, just need to browser to that package, then click on Install button to install the theme.
If you have multiple themes in your store and you would like to choose another theme instead of default theme, let's do the following steps: