In the creative CSS, but I’m not sure how useful department, here is an article showing how to create an array of colors using only modern CSS. I suspect you’d want to combine this with some JavaScript to be able to pick colors, or allow the user to pick colors, which seems to limit its usefulness on its own.

This is a nice article showing several examples where you can use media queries to detect a user’s browser or operating system preferences using CSS.

The Core Framework is a new toolkit for creating and customizing your own CSS framework. There is a UI for generating CSS classes and variables and there is a WordPress plugin. At launch the Core Framework has WordPress extensions for Gutenberg, Bricks, and Oxygen. This video is a first look at the features and a quick test using the Gutenberg and Bricks extensions.

For advanced front-end developers, Kevin Powell suggests not using tabs if you can avoid it, but if you must use them then he does a walk-through showing how to create accessible tabs from scratch. There are more considerations than I realized.

If you are unfamiliar with various CSS units and their use then this introductory article may be useful.

In this in-depth expert-level tutorial Stephanie Eckles provides an overview of modern CSS features and considerations for their use. In the tutorial she shows how the new features of CSS might be used to create a modern brand theme.

There is a new browser Popover API to standardize and simplify creating popovers without needing to use JavaScript. The first implementation will be coming soon to Chrome and Chrome-based browsers.

Of interest for those who believe less is more – In this short video Kevin Powell shows how little CSS you actually need to get a responsive and decent looking webpage.

Block themes lack responsive controls by device size in the Site Editor. In this article Mike McAlister, the developer of the new Ollie block theme, discusses the current intrinsic design / fluid scaling approach and suggests a way that responsive overrides might be implemented.