Fixes the grain visible in the dark theme.
A piece on black was bare trails with nothing between them. The shadow layer is switched off on a dark background — the web version stamps a near-transparent black, which cannot darken black — but that layer is not only shading: in the light theme it washes the whole canvas and fills the space between the trails. Measured over a screenshot, 2.4% of a light piece was untouched background against 23.7% of a dark one.
With nothing in between, and trails 0.35 pixels wide, a dark piece came out as single bright pixels separated by single pure-black ones — the sharpest edge a screen can draw, repeated across the piece, which reads as grain.
On a background too dark to darken, that layer is now painted the other way up: the piece's own two colors, averaged and dimmed to a tenth of their brightness. It goes where the shadows would have gone, so the empty areas of a piece stay at true black and an OLED panel still has nothing to light there. On a device this takes the single-pixel speckle from 2.8% of the piece to 0.7%, with the share that is pure black unchanged.
Light pieces, and the original, sand, rose and forest palettes, are unchanged. ink gains a faint wash of its own.