salvatore - generally that's done by editing your theme. The catch with that is if you edit the theme directly, and then upgrade the theme later to a newer version, you lose your changes.
The easiest way to avoid that is to make a child theme: http://codex.wordpress.org/Child_Themes
Basically you'd make your own 'pilcrow-child' theme. Then you'd want to add this to your stylesheet:
The other way, which may actually be even easier, is to install a plugin that lets you edit the CSS safely. The plugin Jetpack has this ability.