Constant OVERRIDE_ENV 

pub const OVERRIDE_ENV: &'static str;

TILO_FONT_CATALOG: the catalog to read instead of the compiled-in checkout path, for a binary built somewhere else and run here. Without it such a run silently sets every frame in the host’s faces, which looks almost right and is wrong in a way that only shows up in the review of a screenshot. The toolkit reads it into HostPaths::font_catalog, which is where a reader takes it from.