Hi Alexander,
Greetings from Syncfusion.
We have analyzed your requirement and you can achieve this by changing Stroke in SfSunburstChart control as per the below code snippet.
Also, if you want to change the size the stroke, you can achieve this by setting StrokeThickness in chart.
Code snippet
<sunburst:SfSunburstChart ItemsSource="{Binding Data}" Palette="LightCandy"
Stroke="Yellow"
ValueMemberPath="EmployeesCount">
<sunburst:SfSunburstChart.Header>
.. |
Screenshot:
Hope it helps you.
Thanks,
Muneesh Kumar G.