With respect to styles the main changes I see are that
- a consistent naming convention is applied using BEM
- there’s ever more custom properties that are applied consistently
To me this makes customizing Discourse so much easier and accurate. But I guess it depends on your working knowledge. I could imagine there’s a steeper learning curve now for folks that just want to make some adjustments and are not that familiar with CSS.
About documentation, you can check out the Styleguide and I guess the easiest way to scan which custom properties are available is using the dev tools of your browser. Documentation is also being re-worked. Right now there’s two sections in Developer Guides, Themes & Components and Interface. But there’s a huge range between just wanting to declare some custom styles and authoring a component. Some of this is probably buried too deep between developer topics? 