use Google Maps custom styling for the WordPress Flexible Map plugin -- see http://flexible-map.webaware.net.au/manual/styled-maps/

flxmap-custom-styling.php

<?php
/*
Plugin Name: Flxmap custom styling
Plugin URI: https://gist.github.com/webaware/3808eb1cac17cb32a294
Description: use Google Maps custom styling for a map
Version: 1.0.1
Author: WebAware
Author URI: http://webaware.com.au/
*/

/**
* register custom map types with WP Flexible Map plugin
*
* references and examples:
* @link http://flexible-map.webaware.net.au/manual/styled-maps/
* @link https://developers.google.com/maps/documentation/javascript/styling
* @link https://developers.google.com/maps/documentation/javascript/examples/maptype-styled-simple
* @link http://gmaps-samples-v3.googlecode.com/svn/trunk/styledmaps/wizard/index.html
* @link https://snazzymaps.com/
*
* @param array $mapTypes    registered custom map types, keyed by mapTypeId
* @param array $attrs       shortcode attributes for current map
* @return array
*/
add_filter('flexmap_custom_map_types', function($mapTypes, $attrs) {
    // nothing to register if map type isn't specified
    if (empty($attrs['maptype'])) {
        return $mapTypes;
    }

    // check for map using custom map type
    // https://snazzymaps.com/style/18/retro
    if ($attrs['maptype'] == 'flexible1' && empty($mapTypes['flexible1'])) {
        $custom_type = '{
            "styles" : [{"featureType":"administrative","stylers":[{"visibility":"off"}]},{"featureType":"poi","stylers":[{"visibility":"simplified"}]},{"featureType":"road","elementType":"labels","stylers":[{"visibility":"simplified"}]},{"featureType":"water","stylers":[{"visibility":"simplified"}]},{"featureType":"transit","stylers":[{"visibility":"simplified"}]},{"featureType":"landscape","stylers":[{"visibility":"simplified"}]},{"featureType":"road.highway","stylers":[{"visibility":"off"}]},{"featureType":"road.local","stylers":[{"visibility":"on"}]},{"featureType":"road.highway","elementType":"geometry","stylers":[{"visibility":"on"}]},{"featureType":"water","stylers":[{"color":"#84afa3"},{"lightness":52}]},{"stylers":[{"saturation":-17},{"gamma":0.36}]},{"featureType":"transit.line","elementType":"geometry","stylers":[{"color":"#3f518c"}]}],

            "options" : {
                "name" : "Retro"
            }
        }';
        $mapTypes['flexible1'] = json_decode($custom_type);
    }

    return $mapTypes;
}, 10, 2);