If you are updating the CSS on the inventory and are doing more than just changing a few colors you will want to use an external CSS. This will allow you to make changes that don't get overwritten when the plugin gets updated.
Note: This 'Styles' function applies to the dynamic inventory of the plugin only. For the shortcode displays you will need to use a CSS plugin like in 'Jetpack' or 'Scripts n Styles'.
Copy existing CSS from CarDealerPress
If you are going to upload a style sheet as described above, and are just extending one of the existing CarDealerPress themes. You can find the original style sheet in the plugin as shown below. This will give you a good starting point.