In ArcMap, I could create custom polygons to use as callout boxes for my labels. For example, I used a gradient fill to create a two-color background callout box (see pic below). Is this possible to do in ArcGIS Pro, too?

I tried creating a two-color gradient fill polygon symbol in Pro, saving it to the gallery, and then adding it as the background symbol for a callout box. However, it only shows one of the two colors in the labels.