I don’t see where you added it in the HTML you gave us above. You should add it to the first “marker” div. The example at the end of the instructions is intended to show you how to add multiple class names to an element. You add them to the same class attribute but you separate them with spaces.
Please try again and if it doesn’t work then paste your updated HTML in here so we can see what you did. To display your code in here you need to wrap it in triple back ticks. On a line by itself type three back ticks. Then on the first line below the three back ticks paste in your code. Then below your code on a new line type three more back ticks. The back tick on my keyboard is in the upper left just above the Tab key and below the Esc key.