This article has a few interesting design tips, such as ways you can leverage browser features to improve user experience.

CSScrafter is a collection of CSS snippets that you can easily apply to Gutenberg blocks.

In this short video Kevin Powell shows how to use CSS Grid to help with absolute positioning of elements.

This article covers several CSS features with a good explanation for each. For example container queries.

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.