Let's say I have four sub-regions across the page, and that all end up having different heights.

We could set an explicit height, but I wanted to take advantage of flexbox properties, as eluded to in Maxime's Kscope18 presentation, though I didn't find an example in the associated demo app.
In this, he suggested the following CSS
.container {
align-items: stretch;
}
could be used to manipulate regions accordingly. In this case, I'd like to stretch the region heights to fit.

However, I'm having trouble working out exactly how/where to apply this.
I've applied my class to the parent region with the property above to Appearance -> CSS Classes, as per Sven's example, and a few other variations as well - such as applying to the children.

This is with 18.2 UT
Cheers