The §7.9 terminal-default chain resolves host-side: reported theme > createTerm defaultTheme > a black background and a white foreground. The two packed colors ride the §4.2 capability channel into the WASM instance (set_capabilities gains default_bg/default_fg) and are re-pushed when theme folds change them; theme changes emit no bytes and take effect on the next transaction. The renderer consumes them when compositing lands.