Apply a line symbol to a polygon outline in ArcGIS Pro

Summary

It may sometimes be preferable to use line symbols from the ArcGIS Pro gallery to outline polygon feature classes, as shown in the image below:

Procedure

This can be done using the advanced Structure functionality from the Symbology pane.

1. Ensure that the polygon layer is selected from the Contents pane.

2. Click the Feature Layer tab and then click the Symbology button to open the Symbology pane, as shown in the next image.

3. Select the Single symbol primary symbology, click the symbol icon to open its properties, and click the Structure tab.

Note: 
For this example, the Single Symbol primary symbology was used, but the same steps work with other types of symbology.

4. Delete the outline symbol from the Layers list, as shown in the next image.

5. Click the Add symbol layer button and select the Import layers from a symbol option as seen in the next image.

6. The Import layers from symbol window displays. In the Symbol type drop-down menu, select the Line symbol option and choose the desired line symbol from the gallery.
7. Click Import to save the changes.

8. Apply the changes in the Symbology pane. As a result, the polygon outlines have the desired line symbology, as shown in the image below: