Overwrite or extend tiny mce styles dropdown in required-starter child theme.

functions.php

<?php

/**
 * Change options in the TinyMCE Styles dropdown
 * @param  array $args default styles array as seen in req-mce.php
 * @return array       our custom styles for the dropdown
 */
function req_starter_editor_stlye_args_filter( $args ) {

    $custom_args = array(
        array(
            'title'   => __( 'Notes', 'required-starter' ),
            'block'   => 'div',
            'classes' => 'note-paper', // Add your classes here
            'wrapper' => true
        ),
        array(
            'title'   => __( 'Lead-Text', 'required-starter' ),
            'block'   => 'div',
            'classes' => 'lead', // Add your classes here
            'wrapper' => true
        ),
        array(
            'title' => __( 'Image', 'required-starter' ),
            'block'   => 'figure',
            'classes' => 'content-img', // Add your classes here
            'wrapper' => true
        ),
        array(
            'title' => __( 'Partner Logos', 'required-starter' ),
            'block'   => 'div',
            'classes' => 'partner-logos', // Add your classes here
            'wrapper' => true
        ),
        array(
            'title' => __( 'Button', 'required-starter' ),
            'selector'   => 'a',
            'classes' => 'button', // Add your classes here
            'exact'    => true
        ),
    );

    // Since you only have custom styles,
    // we throw away the default $args array
    // and simply return your styles.
    return $custom_args;

    // The alternative would be to use array_merge to
    // combine the original $args and our $custom_args
    //return array_merge($args, $custom_args);

}
add_filter( 'req_editor_stlye_args', 'req_starter_editor_stlye_args_filter' );