Although the plugin comes with 5 different inventory themes out of the box, we understand that you may want something a little more custom. Depending on how extensively you want to customize the theme there three paths we recommend.
1) Add some CSS using a plugin like "Scripts n Styles" or to your sites child theme. The idea here is to make sure and add your custom CSS to an area on the site that you control and that won't get overwritten while updating the CarDealerPress plugin or your site's theme.
This path works great if you are just changing a few colors or some minor styling of one of the existing themes.
2) Create your own custom style sheet. The CarDealerPress plugin has the ability to read an external style sheet from s3 or other server. You would use this route if you want to make significant changes to one of the existing themes by moving elements around etc.
3) Create your own theme. This is when you want to add or change functionality and styling within the inventory. You could start with a theme that is close to what you want and then edit it from there. To make this work in a way that doesn't overwrite your custom inventory theme during updates we have created an "extender" plugin. You will need to upload this plugin to your WordPress website in the plugins folder and then add your theme to the top level of this plugin. Make sure and read the instructions in the README file within the extender plugin.