Instructions for modification on sites created after March 2015 (V5). See below for V4 instructions.
1. This is done on the Styles section of your site, which is accessed from the editing dropdown in the top navigation bar (by default, the dropdown says Live Site).
2. Once you do that, you will be brought to the Styles overlay, which is where you will can build templates and access the custom CSS panel.
3. Choose the Style you will work with.
4. Choose the "CSS" button
5. Add your Custom CSS in the panel, preview, and save changes when you are done.
**HINT: Create a new style if you want to create and work on a site that is not live.
Instructions for modification for sites created before March 2015 (V4).
As long as you have that, you can use custom CSS on the Admin Menu -> Site Style -> Edit CSS -> Edit Custom CSS area of your site.
From here, you can either paste CSS directly (using Option 1 in the image above) or upload your own customized CSS file using Option 2. :)
If you're going to be editing the CSS of your site, I strongly suggest either using the Developer Tools built into your browser, or Firebug (an addon for Firefox).