Timothy Bowers on "[Theme: Limelight] Icons on theme"

There is a chance that the theme has a CSS field in the options but I haven't looked, if not then you can make a child theme:

http://codex.wordpress.org/Child_Themes
http://premium.wpmudev.org/blog/create-wordpress-child-theme/

Or you can instead use a plugin if you like:

http://wordpress.org/plugins/custom-css-manager-plugin/screenshots/

http://wordpress.org/plugins/my-custom-css/screenshots/

http://wordpress.org/plugins/pc-custom-css/

http://wordpress.org/plugins/imporved-simpler-css/screenshots/

Best not to edit directly, doing this would result in changes being lost upon every update unless you're careful. :)

Have a fantastic day! :)