WordPress Checkout plugin use a transparent layout.
- The plugin insert a few styles to the components added to your blog. That way you product labels and checkout stages uses your blog layout and match perfectly any theme.
- If the html added by the plugin to the pages needs to be changed or you need a particular functionality, the default presentation can be easily changed or you can create a new one and use it without affecting any functionality. (In order to do that should know some html and css)
- In your theme you can also add styles to customize the look and feel of the plugin.