What is so special about this really?
You are changing the ↗ hsl values of a $primary-color variable in SCSS.
Which is used in the background color here.
The components (hue, saturation and lightness) of this SCSS $variable are actually CSS --variables!
Hence you can change them on runtime (check your devtools!).
The trick?
Tricking SCSS functions using CSS variables via string concatenation.
Explore the code in ↗ codepen