If you wish to brush up your Flexbox skills, take a look at the interactive guide created by Josh Comeau (not affiliated with Pinegrow).
The guide has a bunch of interactive experiments and covers various Flexbox gotchas.
(Click on the image or on the link below to open the guide)
The best thing about Pinegrow is that it lets you use all your HTML & CSS skills in an open way.
Pinegrow has rich Flexbox controls in the Style panel for CSS, and in visual controls for Bootstrap and Tailwind CSS: