Thank you for the question, I'm sorry you're having trouble with no rules defined in <div class>.

When you see the error, "No rules defined in <div class>," what steps did you take that led to this error? What were you trying to accomplish when you saw it?

Generally speaking, CSS classes will need to have the tag look something like, 

<div class="your-div-class">

And once you have a class defined, you can add CSS rules in your Customize -> Advanced -> Custom CSS & JS menu.

I hope this helps, let us know some additional context so that we can help further.