Styling charts works a bit differently in Mendix. You can follow this howto to style any chart through a json file in your theme folder.
However, that will not let you use the SCSS brand variables. If you really want to use these brand colors from Atlas UI, you could target the HTML elements which should change color and put that styling in `theme/web/sass/app/custom.scss`. Keep in mind, that these HTML elements are SVG, which means the CSS properties are called different sometimes. Example: `background-color` is called `fill` for SVG elements.