CSS + Cookies: A Perfectionist's Approach to Theming in the World of Server side Rendered Apps - Julia Qiu (Zillow/Trulia) Theming using CSS variables and :root pseudo-class yields a much simpler solution than hacking your way to achieving it with CSS in JS. In this talk, we will explore how we can make theming work flawlessly with server-side rendered apps while respecting system defaults.