...which does not work?
Hi.
It works, you just use wrong syntax.
Add your own class in General Tab.
Then, you can use this field.

But ... I would not recommend using this field for production. Only for fitting code. And then transfer it to custom.css file. Since the code that added to this field is placed inside the body in the <style></style> tag, that is not valid and impairs performance.
And one more nuance. In this field you can only use the code for your class and its child elements. You can not refer to the addon wrapppers.
In order to understand which items you can use there, open the code inspector and find your class.

In different addons it may be differently.