Currently, although the ConicalGradientBrush can be added as a Shader; it cannot be customised due to the "Add GradientStop" button not working.

Repro Steps:

1) Create a new Studio Project containing a MainPage.xaml

2) Select the RootGrid, and in its Properties Panel, click on the 'Background' Property, which will open its Brush Picker.

3) From the Brush Picker, select the 'Image Brush' tab, and under the 'Shader' sub-Property, select 'ConiGradientBrush'.
This will open an expander under the SubProperty with a label of GradientStop (0 items).

4) Click on the '+' button directly to the right of the sub-Properties label to add a GradientStop.

Observe how no result occurs, and GradientStops can't be added to the Shader.

Expected Results: GradientStops can be added to the ConicGradientBrush Shader.