It's a great way to make conditional styles without having to use JavaScript; however, having used JS for years to make theme color and icon sets that rely on CSS properties, I'm not sure I particularly like this method. I feel like you have to smear a lot of logic across your CSS whereas with JS you can reduce your theme to a data structure and just have a simple function to setup all the CSS variables based on that.
Am I just an old man?
I like making static informational pages and don't know the first thing about JavaScript, so this could be handy for me.
Just use SCSS to smear the logic across CSS automatically.
Javascript always suffer from FOUC problem though (Unless it's server side). Although the if() css function seems to just be syntax suger of standard @media query. So it doesn't really add anything to solve existing problems.
Edited: It seems it can also be toggled from css variable? So it might actually fix some existing problems.