Supporting CSS variables would:

• Support CSS variables as theme option #12827 (comment): open up avoiding flickering of dark mode websites implemented in frameworks like Next.js or Gatsby! (This seems to be the case on the https://material-ui.com/ website itself! Eg. if you set dark mode and refresh, the page will briefly show up in light mode, and then change to dark mode, causing a flicker)
  Problem demonstrated: https://joshwcomeau.com/css/css-variables-for-react-devs/#dark-mode-flash-fix
  Solution with CSS variables: https://joshwcomeau.com/gatsby/dark-mode/
• [styles] Memoize makeStyle result to avoid rerendering #25018: make the switch between dark and light feel more instant. Relying on CSS cascading will always be faster than React context cascading.

Benchmark

• https://react-spectrum.adobe.com/react-spectrum/theming.html