There are a number of ways you can do this, one of which is to browse to your theme.scss file and click the Default button. In your cloned theme style window (Cascade in your case) you'll see all the headline tags (h1, h2, h3, h4) are scoped under the #body tag. So as an example, to make all h2 headlines italicized, click Current to go back to your current theme.scss file, and add this to the bottom:

#body {
  h2 { font-style: italic; } }