Changing Default Theme .less files

I’m new to expressionengine and I’m trying to learn my way around from square one.

I’m trying to tinker with small changes to the .css file for the default theme and I’m coming against a few road blocks.

I stumbled across this post here: https://expressionengine.com/blog/introducing-the-default-theme

It states that you can make simple changes to the look and feel of the default theme by modifying the .less files, but when I make simple changes, they don’t seem to be reflected on the default site. I’ve only made simple modifications, like changing the primary color from blue to green, but they don’t seem to compile or make any changes whatsoever to the default.min.css file.

Is there an extra step somewhere that I’m omitting by accident? Are the .less files a holdover from a bygone age? Any help would be appreciated.