Hello,
You can try this https://forum.mendix.com/link/space/ui-&-front-end/questions/109705
Hope it helps!
If you are using the default Pie Chart widget from Mendix (based on Plotly), you can change the background of the pie chart in the widget configuration on the Advanced Tab > Layout options (doc).
Using something like
{
paper_bgcolor:"black"
}
should work (Plotly documentation here). I haven't tested this, so perhaps plot_bgcolor works as well.
EDIT: Mendix actually gives the exact example here.