Scrimba (the site that has the course) is really neat. I recently did their CSS Flexbox course and it's just really cool.
It's basically an interactive screencast where you can stop the teacher at any time and play around with the code that they're writing. No more stupid "set up your environment" or needing to have two monitors.
It's easier to show with a gif:
Disclaimer: I know the founders of Scrimba
Nice article to start with custom properties!
I published an article about defining css api using custom properties.