As of the 3.19.5 release, you can add pre-defined styles to individual Charts in the Slate Editor.
Slate Styles are a great way to enhance your story telling experience. Refer to the Slate Styles section in the docs to learn how to build new styles or create your own.
To edit the styles of a Chart, you'll simply click on the Chart and then click on the Slate Styles icon. A modal will appear that allows you to add the styles available to you.
You can also add conditional Slate Styles to your Charts by leveraging the familiar ServiceNow condition builder. Set the conditions you'd like, with an Operator and Value to apply that style to values that fulfill the conditions you've set.