Download composite for free at saberwp/composite: Composite WordPress theme. (github.com) Composite requires the ACF Pro (Advanced Custom Fields) plugin to be installed and active.
Composite is a mixture of old and new, defined in the Gutenberg era of WP as a "classic theme" or "hybrid theme". We continue to use tried and true technologies in this theme including PHP page templates and WP menus. At the same time we're exploring the integration of modern JS and CSS including React and Tailwind.
If it were up to us there would be a field system baked into WordPress. There is a meta field system that is in WordPress core, and ACF Pro along with other field plugins utilize this system to store key/pair data. Unfortunately the WP core only provides the storage, and not the UI. If we put our development time into building the fields for settings and configuration, as well as supporting customization, then we would overwhelmed by this work and largely unable to build the more innovative features we want to provide through Composite. While it is somewhat annoying to have a dependency, we also know that ACF is used as a de facto standard solution for fields by millions of WordPress users.
Using ACF's powerful field system and Gutenberg block building tools has enabled us to minimize the time spent on reinventing the wheel.