Hi,

You need to use child theme to add it, take a look:

http://codex-themes.com/thegem/documentation/css

Child theme in attach

Example:

function theme_js() {
    wp_enqueue_script( 'theme_js', get_stylesheet_directory_uri() . '/js/custom.js', array( 'jquery' ), '1.0', true );
}
add_action('wp_enqueue_scripts', 'theme_js');

Hi,

All the css changes should be done in the style.css of the child theme.

If you need to add your custom.js code, use my function to connect custom.js file in the child theme, (so you need to add the file /js/custom.js and put your JS code there)

Or you can use Custom CSS filed and Custom JS field in the theme options, here:

You're welcome!