To view available basemaps: 
  1. Select the Layers Menu.
  2. Click on Basemaps to open the full dropdown list.  

Click on the desired Basemap.  If you have a custom basemap (for example, your own custom imagery for your agency), it will be the first item in the list.  If you select a different basemap in one workspace, it will persist across to another.

NOTE: To adjust basemap opacity, use the slider option to the right of the basemap dropdown.

Click Basemaps available in Intterra to see what's generally available.

Here are three examples:

Dark Gray Canvas Vector

Imagery with Labels