public static CIMSymbolReference MakeSymbolReference( 
   CIMSymbol symbol
)

{
    CIMPolygonSymbol polySymbol = SymbolFactory.Instance.ConstructPolygonSymbol(ColorFactory.Instance.RedRGB);

    //Get symbol reference from the symbol
    CIMSymbolReference symbolReference = polySymbol.MakeSymbolReference();
}

{
    //Note: Needs QueuedTask to run
    //Get simple renderer from the feature layer
    CIMSimpleRenderer currentRenderer = featureLayer.GetRenderer() as CIMSimpleRenderer;
    if (currentRenderer == null)
        return;
    //Get symbol from the SymbolStyleItem
    CIMSymbol symbolToApply = symbolStyleItem.Symbol;

    //Set symbol's real world setting to be the same as that of the feature layer
    symbolToApply.SetRealWorldUnits(featureLayer.UsesRealWorldSymbolSizes);

    //Update the symbol of the current simple renderer
    currentRenderer.Symbol = symbolToApply.MakeSymbolReference();
    //Update the feature layer renderer
    featureLayer.SetRenderer(currentRenderer);
}

{
      //Get the ArcGIS 2D System style from the Project
      var arcGIS2DStyle =
Project.Current.GetItems<StyleProjectItem>().FirstOrDefault(s => s.Name == "ArcGIS 2D");

      //Note: Needs QueuedTask to run
      //Search for the symbolName style items within the ArcGIS 2D style project item.
      var items = arcGIS2DStyle.SearchSymbols(StyleItemType.PointSymbol, "Circle 1");

      //Gets the CIMSymbol
      CIMSymbol symbolToUse = items.FirstOrDefault().Symbol;

      //Get the renderer of the point feature layer
      CIMSimpleRenderer renderer = featureLayer.GetRenderer() as CIMSimpleRenderer;

      //Set symbol's real world setting to be the same as that of the feature layer
      symbolToUse.SetRealWorldUnits(featureLayer.UsesRealWorldSymbolSizes);

      //Apply the symbol to the feature layer's current renderer
      renderer.Symbol = symbolToUse.MakeSymbolReference();

      //Apply the renderer to the feature layer
      featureLayer.SetRenderer(renderer);
  }

{
    List<string> fields = new List<string>() { "Field1" };
    //Note: Needs QueuedTask to run
    StyleProjectItem styleToUse =
      Project.Current.GetItems<StyleProjectItem>()
          .FirstOrDefault(s => s.Name == "ColorBrewer Schemes (RGB)");
    if (style == null) return;
    //Note: Needs QueuedTask to run
    var colorRampListFound = styleToUse.SearchColorRamps("Red-Gray (10 Classes)");
    if (colorRampListFound == null || colorRampListFound.Count == 0) return;
    CIMColorRamp cimColorRamp = null;
    CIMRenderer renderer = null;

    cimColorRamp = colorRampListFound[0].ColorRamp;
    var rendererDef = new UniqueValueRendererDefinition(fields, null, cimColorRamp);
    renderer = featureLayer?.CreateRenderer(rendererDef);
    featureLayer?.SetRenderer(renderer);
}