How to add a specific body class for the dark (or light) mode?

Thanks for clarifying, I understand now.

We do not add any class to the body based on the user’s currently selected color scheme.

If I can probe further, is the reason being that you dislike the color our color functions generate for --tertiary on your currently selected dark scheme?

What would you like to achieve through a body class that you feel like you are not able to do currently?