VisualElements are defined in code, and any sub-elements created by the class in its constructor are shown as read-only (disabled) in the UI Builder.
Through the UI Builder and UXML you can only add children to that element's content container, a child element defined by the class.
You cannot add inline styles to read-only elements, use USS selectors to style the element.
To style an element globally, target its classes with a simple selector.
To style an element locally, create a complex selector targeting from the parent element you can add a name or custom class to.