First, you need to enable the Custom CSS plugin for your site if it isn't already enabled. (Enabling Plugins)
Once the plugin is enabled:
- Login to your site
- When viewing your site (not the dashboard) a link labelled Edit custom CSS should appear in the admin bar, click this.
- A pop-up window will appear where you can input CSS. You can see your changes instantaneously on your site as you modify the CSS code.
- Don't forget to save your changes when you're done.
If the pop-up causes problems, you can also edit your custom CSS in the dashboard:
- Go to Dashboard -> Appearances -> Custom CSS
- Add your code to the Edit CSS field
- Save Changes and then view your site to ensure your CSS has the desired effect.