One thing you can try is the place where you gave the fill, give it s text-shadow with an !important tag to make it more specific CSS.
However this is not the right solution. The Widget should give you the custom styling options. I know that above method is just overriding CSS on a situational basis but the right solution is to find the custom styling options provided by the underlying plottly.js (which drive the AnyChart widget)
Can you check if the widget gives you more customization options?