Change font-size of av-subheading, av-subheadingbelow

Hi,
Thank you for tihs great theme.
I have a quick question.
I want to change the font-size of this div-class : av-subheading and av-subheadingbelow.
I’ve tried to change this in the quick css like that :

.av-subheading_below {
  font-family:'open sans';
  font-size:20px;
  font-style:italic;
  margin-top:2.3em;
}

All the changes are ok (font- family, font style et margin-top) except for du font-size. It doesn’t work for the font-size, the value is still 15 px.

Can you help me please ?

Big thank you !