Show the benefit: "theme.json replaces a dozen add_theme_support calls with one file. I define colour palettes and font sizes there, and WordPress generates CSS variables and restricts the editor to those choices — consistent design without custom CSS."
theme.json is a configuration file that controls the block editor experience and global styles. Settings: define colour palettes, font families, font sizes, spacing presets, layout widths, and which block features are enabled or disabled (custom colours, gradients, padding). Styles: set default typography, colours, and spacing for the entire site and individual blocks. It replaces add_theme_support() calls and editor-style.css with a single declarative file. theme.json generates CSS custom properties automatically. Strong candidates explain: the schema version, that theme.json works in both classic and block themes, the settings versus styles distinction, and that it controls what options editors see in the block editor sidebar.
Tests modern WordPress knowledge. Candidates who do not know theme.json are missing the central configuration mechanism for block editor themes. Those who understand it can build themes that give editors controlled flexibility.