A listing setting allows you to adjust your pricing and inventory to be different to that of your store. You may wish to have your prices 10% higher than your store, or display that you have fewer items in stock on the marketplace.
You can apply 1 setting to many products, so that all products follow the rules setup within that Listing Setting.
eBay requires a Listing Setting to list items, since we need to know all required aspects of a listing like duration, shipping methods, payment methods etc.
Amazon is optional since the majority of settings are created within Seller Central. The main purpose of an Amazon setting is to affect pricing, or specify if your items are FBA.
A listing theme is used for eBay only and is a way of customising the layout of your listing page. We have a full html editor allowing you to create your own theme, import one from a third party, or alter our default pre-created themes.
The following pages go into more detail.