Constant THEME_CSS_BASENAME 

pub const THEME_CSS_BASENAME: &str = "relaxng-schema-rustdoc-theme.css";

Filename of the rustdoc-styled theme stylesheet that ships next to each schema-doc site. Auto-prepended to --css when --schemadocs is on, so callers don’t need to remember it. The orchestration shell copies the source CSS into $output_dir under this same basename so the relative <link> resolves.