You can also do it how you were doing it, just don't "change" the .row, add "to" the .row so it would be:
.row .intro-row
add your attributes to the Class in the Styles window for that .intro-row and you should be all set then. Basically a few ways different ways to do the same thing.
I personally prefer to use the attributes box in the HTML window, quicker and easier and less confusing as you won't accidentally end up with classes inline as much as you would by using the new Options area for classes.