add_action in class and use it in theme

I recommend to add a static method to access the plugin instance.

Example, taken from this answer:

class My_Plugin
{
    private $var="foo";

    protected static $instance = NULL;

    public static function get_instance()
    {
        // create an object
        NULL === self::$instance and self::$instance = new self;

        return self::$instance; // return the object
    }

    public function __construct()
    {
        // set up your variables etc.
    }

    public function foo()
    {
        return $this->var;
    }
}

// create an instance on wp_loaded
add_action( 'wp_loaded', array( 'My_Plugin', 'get_instance' ) );

In your theme you can access foo() now like this:

print My_Plugin::get_instance()->foo();

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do_action() never returns something. You would need apply_filters():

$footer1 = apply_filters( 'finaboo_footer', 1 );