frostPermille property
Glass: how much of the backdrop the glass diffuses × 1000 (0 = clear, 1000 = the solid tint). The renderer lays the background's base colour under the tint at this opacity, so the face is still the tint composited over a point of the background's colour hull (what the guardrails sample). Liquid Glass only; serialised only when set, so every other theme's canonical JSON and stable hash are unchanged.
Implementation
final int frostPermille;