Theming using css vars

[harshvardhanx007]

I was looking into css variables which was introduced not long time back. But it lacks example of it's usage or any recommended way to use it. Do anyone have idea how we can implement multiple theme like dark, light where light theme can have multiple color palettes in itself. If possible I want to avoid use of react lifecycle methods or css-in-js, it should be more like I can just modify classname on root and the theme changes.