Day 3 was a nice intro to CSS variables.
These will be super powerful once they are more widely adopted. Unfortunately they don't work in IE or Edge currently so we've got a bit more time until we can start using them in production. According to http://caniuse.com/#feat=css-variables they've had support in Chrome since 49 and Firefox since 31. Safari, Opera, iOS Safari, and Chrome for Android also are supported currently. MS Edge is working on it and I doubt we'll ever see it in IE 11.
Still though, the power to change a single CSS variable and have it update every declaration that uses that variable automatically will probably change the way that styles are written.
CanIUse.com shows about 68% of users currently have access to this feature and I'd expect that to go up dramatically over the next couple years.
Completion Time: ~25 Minutes