Line symbology having casing (background), when published as a hosted feature service and viewed in ArcGIS.com viewer, the casing pre-dominates the actual symbology.
- Navigate to the edit symbol for the line feature and remove the background layer and republish this map to <a href="http://ArcGIS.com" target="_blank">ArcGIS.com</a> this will display absolutely fine inside <a href="http://ArcGIS.com" target="_blank">ArcGIS.com</a> map viewer.