Function fill_scrim 

pub fn fill_scrim(
    scene: &mut Scene,
    transform: Affine,
    rect: Rect,
    strength: f32,
    theme: &Theme,
)

Paint a scrim over rect at strength, in the theme’s own scrim hue.

A flat wash dims by an amount that depends on the ground’s own lightness: measured on a real frame, a flat 0.45 wash took 39.6 L* off a bg-white page (252→143) and 1.5 L* off the desktop beside it (20→16) — the page read as blanked out and nothing else read as dimmed — and flattened the 12 code values between the page and a rule on it down to 7.

So this is two fills instead of one: a soft-light tone stage (Cb² against black, a proportional move that dims the middle/bottom without flattening the top) and a flat level stage in the theme’s scrim hue at SCRIM_LEVEL_SHARE of strength. Together they take a uniform 21-23 L* off every ground from #ffffff to mid grey. Blend modes need a backdrop, so call this after whatever it dims is already drawn into the same Scene.