Custom styles in Tiny MCE with an external CSS file

The path to the CSS file tinymce.min.css was incorrect. The solution was to change:

$url .= trailingslashit( plugin_dir_url(get_stylesheet_directory_uri()) ) . '/tinymce.min.css';

to:

$StyleUrl = get_stylesheet_directory_uri().'/style-sheets/tinymce.min.css';

I also had to change:

$StyleUrl = plugin_dir_url(get_stylesheet_directory_uri()).'tinymce.min.css';

to:

$StyleUrl = get_stylesheet_directory_uri().'/style-sheets/tinymce.min.css';

Now all I had to do was to remove the styles array so the complete code is:

// Apply styles to the visual editor
add_filter('mce_css', 'tuts_mcekit_editor_style');
function tuts_mcekit_editor_style($url) {

    if ( !empty($url) )
    $url .= ',';

    // Retrieves the plugin directory URL
    // Change the path here if using different directories
    $url .= trailingslashit( get_stylesheet_directory_uri() ) . 'style-sheets/tinymce.min.css';
    return $url;
}

// Add "Styles" drop-down
add_filter( 'mce_buttons_2', 'tuts_mce_editor_buttons' );
function tuts_mce_editor_buttons( $buttons ) {
    array_unshift( $buttons, 'styleselect' );
    return $buttons;
}

// Add styles/classes to the "Styles" drop-down
add_filter( 'tiny_mce_before_init', 'tuts_mce_before_init' );
function tuts_mce_before_init( $settings ) {

    $style_formats = array(
    array(
        'title' => 'Emphasis box',
        'block' => 'p',
        'classes' => 'emphasis-box'
        ),
    array(
        'title' => 'Alert box blue',
        'block' => 'section',
        'classes' => 'alertbox blue',
        'wrapper' => true
        ),
    array(
        'title' => 'Alert box yellow',
        'block' => 'section',
        'classes' => 'alertbox yellow',
        'wrapper' => true
        ),
    array(
        'title' => 'Alert box red',
        'block' => 'section',
        'classes' => 'alertbox red',
        'wrapper' => true
        ),
    array(
        'title' => 'Alert box green',
        'block' => 'section',
        'classes' => 'alertbox green',
        'wrapper' => true
        ),
    array(
        'title' => 'Alert box pink',
        'block' => 'section',
        'classes' => 'alertbox pink',
        'wrapper' => true
        )
    );

    $settings['style_formats'] = json_encode( $style_formats );
    return $settings;
}

// Learn TinyMCE style format options at http://www.tinymce.com/wiki.php/Configuration:formats
 // Add custom stylesheet to the website front-end with hook 'wp_enqueue_scripts'
add_action('wp_enqueue_scripts', 'tuts_mcekit_editor_enqueue');

// Enqueue stylesheet, if it exists.
function tuts_mcekit_editor_enqueue() {
$StyleUrl = get_stylesheet_directory_uri().'/style-sheets/tinymce.min.css'; // Customstyle.css is relative to the current file
wp_enqueue_style( 'myCustomStyles', $StyleUrl );
}