This page allows you to modify:

• Header and footer that will appear in all your frontend pages
  • Web notification pages, connected with subscription and unsubscription processes, configurable for each list under Settings > List settings > Notification pages
  • Profile updates forms
  • Published emails archive
  • "Send to a friend" pages

• the style of standard subscription forms:

You can modify header and footer using the built-in editor, for a quick and accurate job. 

The third box allows you to modify the form CSS: subscription forms are mainly dynamic pages, so they cannot be modified directly, but only using general style rules given by the CSS code.