Function register_multibuffer_theme_elements 

pub fn register_multibuffer_theme_elements(
    reg: &dyn ThemeRegistry,
    owner: ElementOwner,
) -> MultibufferHeaderElementIds

Register the multibuffer mode’s theme elements against reg. Idempotent by name (safe to call on every mode activation / every view creation). Returns the interned [ElementId]s the excerpt-header provider bakes from.

owner is the mode’s id string (MultibufferMode::mode_id → as_str), so :describe-element attributes these to the mode rather than core. lattice-theme is a leaf crate that can’t depend on lattice-mode, hence the string owner.