Thanks for reporting this. It was a bug rather than intended behaviour.
The Chart control selected the correct light or dark colours when drawing, but it wasn’t being redrawn when the system appearance changed. This affected every chart type, not only Pie charts, and wasn’t specific to Linux.
I’ve fixed it for the next release: https://feedback.objo.dev/bug/1352
Charts using the default background will now update automatically when the system switches appearance. An explicitly assigned BackgroundColour will remain fixed, as intended.
Until the update is available, this can be worked around in the Window’s AppearanceChanged event:
Event AppearanceChanged()
Chart1.Refresh()
End Event