Converting styles from ArcGIS Pro (.stylx) to ArcMap (.style)?

The ArcGIS Pro documentation says:

Styles created in previous versions of ArcGIS Desktop can be used in ArcGIS Pro. Importing the style simultaneously creates an updated *.stylx version of the style and adds it to the project.

It is clear that ArcGIS Pro can import styles (*.style) from the older ArcMap architecture but I would not expect ArcMap to be able to import styles (*.stylx) from the newer ArcGIS Pro architecture.

If you need to share styles between ArcMap and ArcGIS Pro then I recommend that you create them using ArcMap as *.style files.

If you have ArcGIS Portal set up you can send the ArcGIS Pro layer to the portal as a web layer. Then through the web browser interface of the portal you can go to the web layer and click "Open in ArcGIS Desktop". That will open the layer file in ArcMap and you can save the styles, or export a layer file from ArcMap that will retain the styles in ArcMap format.