The XYZ Tile source layer option supports Mapbox Studio Styles. So If you have a Mapbox Studio account you can use your Mapbox Styles in Fulcrum for online use on both the Fulcrum web and mobile apps.
To get the XYZ Tile URL, you will want to follow Mapbox's help documentation on obtaining a Share URL. When you are on the Share, develop & use page, you will want to select Fulcrum in the "Use Style in GIS Apps" section.
For more general information on adding layers, please visit our adding layers page.
NOTE: The XYZ Tile source is support for online use on the mobile devices.