The 2.4 release is mostly importer work, which is the least glamorous part of a theme and the part that decides whether the first hour goes well.
Importer
Saved templates and menu assignments now come across with everything else, and a failed step can be retried on its own rather than restarting the whole run.
Header layouts
Every theme in the catalogue picked up a second header layout, switchable from the theme options panel without touching the builder.
Fixes
RTL spacing across all demos, a WooCommerce cart template refresh, and a fix for widget areas importing empty on multisite.