Yes, unfortunately. Most of the chart components are based on AMCharts (but I'm not sure if that includes the Simple Gauge) and they don't understand the CSS variables.
I solved the problem by creating a folder of session variables with the same names as the built-in themes and assigned the hex RGB values to those. Then you can bind the color property to the session variables. It became a bit more of a nuisance when the project had a light and dark theme and I had to create an expression binding on each of the session color variables.