It seems a chart is an SVG image and applying a CSS might not be the easiest and shortest solution.
I see a similar enquiry under ‘Platform Suggestions & Feedback’ for more control over graphs and their colours from your side. A member of the support team will reply asap: More control over colours used in graphs
Here is why I think the CSS route might not be the quickest one: any CSS applied to a chart is applied at a much higher level DIV. As a result it is a little tricky to determine the exact segments of classes which require customisation i.e. from custom classes to individual classes.