Changing size of headings

I’ve changed the CSS code to make the headings different font sizes to the default, but it doesn’t seem to have flown through to my blog posts (see rightbrainrevival.com). Have I missed something? The new code in the style.css file is below:

h1 {
  font-size: 24px;
}

h2 {
  font-size: 20px;
}

h3 {
  font-size: 16px;
  font-style: italic;
}

h4 {
  font-size: 16px;
}

h5 {
  font-size: 14px;
}

h6 {
  font-size: 13px;
}