Function resolve_theme_overrides 

pub fn resolve_theme_overrides(
    base_catalog: &Catalog,
    base_revision: &str,
    overrides: &ThemeOverrides,
) -> Result<ResolvedThemeCatalog, ThemeOverrideError>

Applies user palette definitions over an immutable built-in catalog.

Every extends lookup uses base_catalog, including when the configured name shadows a built-in theme. This makes default extends default an intentional override instead of a recursive definition.