Custom CSS

Hello,
You can always use a plugin for that.
Jetpack for example allows you to add custom CSS to your theme: http://jetpack.me/support/custom-css/

The built-in custom CSS option is available if you download the theme from the author’s website. Or you could use a child theme, create a style.css file and edit it with the theme editor. Or, as @jcbrebion suggested, you could use a plugin.