Adding user css classes

How can I add custom css classes to choose from?
I know it should be explained here:

But the according link: "Adding Custom CSS Classes to Blocks, Areas and the Editor" is broken. Like many others there.

Can someone tell me how to do it?

View Replies:
core77 replied on at Permalink Reply

the docs are there:

I just tried this feature of 5.7 and it's very nice.

1. add your block type and the desired css class into the method in page_theme.php
2. select the class from the concrete-ui. In edit mode click on your block, then design and custom template and then you can select the custom class.