try adding a custom class to the widget and specify a new stylesheet which is more specific in styling then the widget barchart.css.
This will make sure the new styling will be applied. The widget won't update the css based on a property setting.
I think the current version of the ChartJS widget renders almost everything into a canvas element.
That means you cannot change it with CSS;
The ChartJS widget implementation allows only setting of the line color in the Multi or Single SeriesDataset entity. The underlying library of the widget used the same hardcoded #666 color for both font and line colors.