I have followed up how to guide for Configuring Portal for ArcGIS with custom symbology:
and managed to create custom symbology but, it seams that from last step where I updated Symbol Sets Group Query with:
title: "<name of custom symbols group>" AND owner: <group owner user name>
it overrides default symbols (I have all of mine custom symbols but do not have any of default). I can change it back like it was by default with:
title: "Esri Symbols" AND owner: "esri_en"
but is there a possibility to combine these two? To have both Esri's and mine custom symbology?
When doing it this manual way I would share those webstyles to the group you are using. When using the upcoming release of Portal with the released version of Map Viewer the esri styles will automatically get shared to the custom group just like how Online handles this workflow.