How excatly we change the background when we are at different attempt kind?, and how the nested buttons works?

Hi, I need a confirmation about this topic. craig introduced a technique that let us add and remove styles from our app. we use this code to add style:

 private void addAttemptStyle(AttemptKind kind) {
        container.getStyleClass().add(kind.toString().toLowerCase());
    }

i understand that we get the current style from the container and then we add the attemptkind and getting the enum's name(break or focus) from toString and when we get a kind lets say focus, in our css file we have a .focus refernce that change the color, and the question is how javafx knows what is focus if we havent add an id for it in the fxml file?

another thing is that the logic of the fxml file isnt very clear and we havn't dive into that properly, anyone has a external resource or code for learning fxml?

Steven Parker Tonnie Fanadez Eric McKibbin Sean M