I want to mention, that the plugin is designed in a way, that you can’t edit the code from scratch. You can make use of the pre-built themes/options of the plugin and the Custom CSS option where you can write your own CSS code to achieve specific styling results.
I want to mention, that the class is connected to the main container. Most presumably, the selector you wrote was not a strong one, which is why the CSS was not connected.
You can add !important to the CSS, by writing, for example, like this:
#ays-quiz-container-16.my-class p { color: red !important; }