Remove a Plugin function through theme’s function.php?

Same way you would remove it from anywhere else, be it a plugin or theme or whatever.

add_action('plugins_loaded','remove_whatever');
function remove_whatever() {
    if (class_exists('PixGridder')) {
        remove_action( 'wp_enqueue_scripts', array( PixGridder::get_instance(), 'front_styles' ) );
    }
}

Might want to use a better function name than “remove_whatever” though. 🙂