<?php
add_filter( 'rwmb_wysiwyg_begin_html', 'fix_begin_hook' );

function fix_begin_hook( $html ) {
    add_filter( 'tiny_mce_before_init', 'fix_capture_css' );
    return $html;
}

function fix_capture_css( $settings ) {
    // Do not load any css for the editor.
    $settings['content_css'] = '';
    $settings['skin_url'] = 'http://google.com/';

    return $settings;
}

add_filter( 'rwmb_wysiwyg_after_html', 'fix_after_hook' );

function fix_after_hook( $html ) {
    remove_filter( 'tiny_mce_before_init', 'fix_capture_css' );
    return $html;
}