Say the feed page's background before the stylesheet canUnverified

Clicking through to the feed flashed white for one frame on the way
in. The page names an external stylesheet, and a transform's result is
parsed and painted once before that stylesheet has been applied: for
that frame the canvas is the browser's default, which is white
whatever the page is about to be. Coming from a dark page it reads as
the page blanking, and it is the first thing a reader sees of it.

Say the colour inline, in the head the transform writes, where it is
in hand the moment the markup is. The stylesheet still owns the
page - its rule comes later and wins the instant it arrives - and this
only covers the gap before it.

On body rather than on :root, which is where style.scss puts it too. A
background on the root element becomes the canvas and stops the body's
own from ever reaching it, which would quietly cost this page the glow
every other page is painted on.

Both renderings of the page say it, since both have the gap. The two
colours are style.scss's --paper, light and dark, named once per file
now that the theme-color metas and this rule both want them.

Measured on the way in from the home page, screencast frame by frame:
one pure-white frame before, none after, dark and light, in the
transform and in the standby alike.