In reply to Justin Tadlock.

Thank you, Justin!

Let's see if I got it now.

The old way I had to build a template for every sidebar (sidebar-footer.php, sidebar-blog.php and so on). Which was indeed somewhat strange.
Now, in the new era, I will build only one sidebar.php template where I call the IDs directly from the functions.php. I can call this sidebar (with ID) from every template/template-part.

So the point is I can define something in functions.php and access this from inside any template part, right?

I wonder if this new way of accessing data will have any impact on the theme colors conundrum. With that I mean that Gutenberg colors are defined inside functions.php but I cant't really do something with these colors. They just sit there.
I can output some inline CSS but that's about it.

I am constantly looking for a connection of my theme design system (build on SASS) and the Gutenberg color palette. There are workarounds with custom properties, but I haven't found one yet that works for me.