@Paul,

We are letting the theme really set most of the layout and color options for the plugin, What we suggets if you need to make css adjustment to do them in a child theme or the theme css file that way you can update both the plugin and theme without loosing your changes.

The add to cart issue, may be a plugin conflict, try deactivating some of your plugin or use a default theme, we could look at the site but we will have to ask to donate to the plugin