Creating a Mapbox Tileset
Mapbox is a mapping alternative to Google Maps. It uses user-defined to create customized maps depending on the information you need to display.
You can easily use any of the existing stylers by selecting the style, and in the "Develop with this style" box selecting "Leaflet"
Copy this URL in to the map settings in Appearance ▸ Customize ▸ Listings ▸ Map Settings.
Create Your Own Style
To create your own style return to Mapbox Studio and click "New Style". You can choose a base style to work from which will provide default options.
Once your base is chosen you can modify any of the available options. Make your changes in the Studio editor and click the "Publish" button at the top of the page. You can then Publish the existing tileset or create a new set based on these settings.
Once published click "Preview, develop & use" and you will be taken to the start of this guide where you can copy the Leaflet URL and add it to the theme customizer.