Function set_plugin_element_override 

pub fn set_plugin_element_override(
    registry: &dyn ThemeRegistry,
    plugin_id: &str,
    name: &str,
    spec: StyleSpec,
) -> Result<(), String>

TK.5: the set-element-override body — an override for an element this plugin owns, above the theme.

Ownership is checked, not assumed. Namespacing already bounds what a plugin can name, so this check should be unreachable; it exists because “should be unreachable” is exactly the reasoning that makes a security boundary depend on a call site staying correct. Refusing here means a future caller that forgets to namespace is refused rather than allowed to restyle a builtin.