The Ecwid API allows you to easily create custom theme applications with Custom CSS, which can be installed in one click and immediately improve the appearance of a store.
Register your app
Get started by registering your theme and specifying the link to your CSS file in the registration form. Ecwid will load your CSS file anytime a store that installed your app is loaded.
To make your theme available to anyone, please make sure to upload your CSS file to a public resource which can handle many simultaneous requests.
You can also sell your themes to Ecwid users for a fixed monthly fee by using Ecwid Billing. Once set up, you will get 80% of all revenues generated by your app/theme. Check out the Make Money page for more details.
With the Ecwid API you can create a theme with some basic settings, such as color picker, element display toggle and others. To learn how to set this up, check out the Public Application Config and Embedded Apps sections in the documentation.