We're using the Mythic starter theme (documentation).

Our criteria, ranked in terms of importance, was:

1. modern but stable
2. as close to the WP way as possible
3. attainable on any host, even managed
4. support
5. updates

And this article is a good introduction to the theme, showing why we chose it for our project. More on Mythic and its underlying framework.

What is a starter theme? Starter themes are not to be used as parent themes, they’re to be used to create your own parent theme. They're used for stuff that repeats with every theme you build, like the template structure and certain functionality. Here's an article on the difference between a framework/starter theme and parent and child themes like Genesis.

We're using CSS Grid and Flexbox ^#11.

Here are some good resources for learning, in different formats depending on how you learn best:

• [video] https://www.youtube.com/watch?v=DCZdCKjnBCs
• [course] https://cssgrid.io/
• [video/hands-on] https://scrimba.com/g/gR8PTE
• [text] https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Grid_Layout/Basic_Concepts_of_Grid_Layout
• [text/video/hands-on] https://gridbyexample.com/what/
• [text/video/hands-on] https://mozilladevelopers.github.io/playground/css-grid
• [hands-on] https://cssgridgarden.com/
• [course] https://thecssworkshop.com/
• https://css-grid.coffeecup.com/
• [video/hands-on] https://scrimba.com/g/gflexbox
• [course] https://flexbox.io/
• [hands-on] https://flexboxfroggy.com/
• [hands-on] http://www.flexboxdefense.com/