public static CIMSymbolReference MakeSymbolReference( 
   CIMSymbol symbol
)

CIMPolygonSymbol symbol = SymbolFactory.Instance.ConstructPolygonSymbol(ColorFactory.Instance.RedRGB);

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

public Task SetFeatureLayerSymbolFromStyleItemAsync(
           FeatureLayer ftrLayer, SymbolStyleItem symbolItem)
{
  if (ftrLayer == null || symbolItem == null)
    throw new System.ArgumentNullException();

  return QueuedTask.Run(() =>
  {
    //Get simple renderer from the feature layer
    CIMSimpleRenderer currentRenderer = ftrLayer.GetRenderer() as CIMSimpleRenderer;
    if (currentRenderer == null)
      return;
    //Get symbol from the SymbolStyleItem
    CIMSymbol symbol = symbolItem.Symbol;

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

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

  // var map = MapView.Active.Map;
  // if (map == null)
  //        return;
  // var pointFeatureLayer =
  //       map.GetLayersAsFlattenedList()
  //          .OfType<FeatureLayer>()
  //         .Where(fl => fl.ShapeType == esriGeometryType.esriGeometryPoint);
  //   await ApplySymbolToFeatureLayerAsync(pointFeatureLayer.FirstOrDefault(), "Fire Station");

  public Task ApplySymbolToFeatureLayerAsync(FeatureLayer featureLayer, string symbolName)
  {
    return QueuedTask.Run(async () =>
    {
      //Get the ArcGIS 2D System style from the Project
      var arcGIS2DStyle =
Project.Current.GetItems<StyleProjectItem>().FirstOrDefault(s => s.Name == "ArcGIS 2D");

      //Search for the symbolName style items within the ArcGIS 2D style project item.
      var items = await QueuedTask.Run(() =>
      arcGIS2DStyle.SearchSymbols(StyleItemType.PointSymbol, symbolName));

      //Gets the CIMSymbol
      CIMSymbol symbol = 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
      symbol.SetRealWorldUnits(featureLayer.UsesRealWorldSymbolSizes);

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

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

public async Task ApplyColorRampAsync(FeatureLayer featureLayer, List<string> fields)
{

    StyleProjectItem style =
        Project.Current.GetItems<StyleProjectItem>()
            .FirstOrDefault(s => s.Name == "ColorBrewer Schemes (RGB)");
    if (style == null) return;
    var colorRampList = await QueuedTask.Run(() => 
                style.SearchColorRamps("Red-Gray (10 Classes)"));
    if (colorRampList == null || colorRampList.Count == 0) return;
    CIMColorRamp cimColorRamp = null;
    CIMRenderer renderer = null;
    await QueuedTask.Run(() =>
    {
        cimColorRamp = colorRampList[0].ColorRamp;
        var rendererDef = new UniqueValueRendererDefinition(fields, null, cimColorRamp);
        renderer = featureLayer?.CreateRenderer(rendererDef);
        featureLayer?.SetRenderer(renderer);
    });

}