Where do I add custom css for the Redactor editor in the Craft control panel?
I ended up using the Control Panel CSS plugin by Lindsey D to do this. Installed the plugin and then pointed it to an external style sheet with my redactor styles.

Here's the plugin:
https://github.com/lindseydiloreto/craft-cpcss

And in my stylesheet, the styles look like the below. These examples are for a custom item I added to the redactor formatting menu with a class pullquote__quote. The first style the menu item in the formatting dropdown menu. The second style the text in the editor.

.redactor-dropdown .redactor-formatting-p-pullquote__quote {
  font-size: 20px;
  font-weight: bold;
  color: #ffcb00;
}

.redactor-editor .pullquote__quote {
  font-size: 20px;
  font-weight: bold;
  color: #ffcb00;
}


And my redactor config file for this rich text field has this corresponding addition:

formattingAdd: [
    {
        tag: 'p',
        title: 'Big quote',
        class: 'pullquote__quote'
    }       
]