Function outline_glyph 

fn outline_glyph(
    rgba: &[u8],
    width: u32,
    height: u32,
    radius: i32,
    fill: [u8; 3],
    outline: [u8; 3],
) -> Vec<u8> 

Repaint a monochrome glyph’s body to fill and ring it with outline, so it contrasts against both light and dark panels. Glyph-body alpha is preserved (anti-aliased edges stay smooth); the outline ring is fully opaque; everything else stays transparent.