I have been developing custom elements in the past based off the Sample Element and Element API outlined in the Docs: https://theme.co/docs/element-api-primer
With the recent update to 5.x, all Cornerstone styles are no longer generated or outputted to the builder or front-end. This used to be handed by style template callback section of the Custom Element. However, now that all current elements are now using the updated TSS system of CSS generation, it seems the 'style' section of the cs_register_element function arguments is not working / connected up correctly any more.
This can be demonstrated by simply using the basic sample element provided from the Element API Primer page in the docs.
Downloading and installing the sample element to a website, it shows up correctly in the Element list, and can be added to the page, and the text is editable. However, all of the styling options have no effect: Background and Margin.
Either the functionality of the 'style' callback needs to be reinstated, or there needs to be a way to hook in to and add to the TSS system that has been set up for the other elements.
Cheers!