WordPress plugin
You can customize the feed by setting the options on the Customize page, which can be found under the Facebook Feed menu in your left hand WordPress admin menu. If you need even deeper customization than the built in options allow then you can add your own CSS to the plugin in the Custom CSS section under the Misc tab.
You can also override these styles for individual feeds by setting options within the shortcode. For example, you can change the height of a specific feed like so: [custom-facebook-feed height=500px]. See a full list of all shortcode options here.
Standalone version
You can customize the feed by settings the options in the fbfeed-settings.php file. If you need deeper customization then you can add your own custom CSS to your website’s stylesheet to completely customize the way the feed looks.
You can also override these styles for individual feeds by setting custom options which you can pass into the fbFeed() function. For example, you can change the height of a specific feed like so:
$custom = array(
'height' => '500px'
);
fbFeed($settings, $custom);