Azure map reference layer formatting

Hi there, 

I am just trying to use the new Azure map Reference Layer, I have it working as below: 

My question is - how can I format this reference layer so that its not blue? 

Ideally I want to only see the reference layer as - a reference of the boundaries. So no fill and black or white thin boundary lines. 

Can anyone help? I have tried to change the settings in my GeoJson file to no avail. 

Thanks.