How do I customize the element added to the screen with cus

I would like to be able to edit the elements using the css block, but I read here on the forum that this is not possible and it would be a considerable amount of work to do so, but for me, it would work if I could already add the elements to the interface with my css, that is, if I can edit the source code to add an element with its necessary CSS formatting. Is there a specific function that adds elements to the screen, can I adapt it to add elements with my CSS customization? What would be the initial steps I could take to achieve this? What files and functions would need to be edited? Can someone give me some initial guidance?

Updated 04/02

I’ve been reading the documentation better now, and it seems to me that the best way to do it would be either via API or creating an AddOn for it. Which method would be most recommended? Would it be possible to create a file and add shapes to it, with their visual specifications, directly through the API? And one more thing, is the API also available in the downloaded code that I uploaded to Docker?