CSS Customization

Hi there. I’m relatively new to WordPress, but have been successfully using .css and .php files to customize a site using the Customizr template.

I just added this plugin, and the functionality is exactly what I was looking for. However, I’m having a bit of trouble figuring out how to customize its appearance through CSS. Could anyone give me some examples of how to get started there? I just need some help with the syntax. Thanks!

http://wordpress.org/plugins/advanced-custom-fields-nav-menu-field/