New theme_from_dir() creates a theme from a directory of template files, with optional _prefab.yml sidecar for per-file strategy and template data control.
New load_themes() sources custom theme definitions from a file, with support for the PREFAB_THEMES environment variable and a ~/.prefab-themes.R default.
step_file() now accepts data = "auto" as a clearer alternative to data = list() for enabling auto-context template rendering.