Let the reader pick the theme, and spread it from the switchUnverified

The palette flipped on prefers-color-scheme alone, so the only way to
read the site the other way round was to change your machine. There is
now a switch at the right end of the masthead, and the change sweeps out
from it rather than landing all at once.

The icon is one disc wearing two faces. The sun is that disc plus eight
rays; the moon is the same disc with a second circle in an SVG mask slid
over its rim to carve the crescent, while the rays fold away. Both move
with transform only and never an SVG geometry attribute - Safari is
unreliable there, as the note about `d` in baseof.html already records.

What gets stored is an override, not a theme. `data-theme` is absent
until the reader asks for something their OS is not already saying, and
toggling back into agreement removes it rather than pinning the same
value, so a machine that turns dark at sunset still carries the site
with it. Every dark rule therefore has two halves - "the OS says dark
and nothing overrides it", and "the reader pinned dark" - and writing
that pair out by hand in four places is how the two drift apart, so the
`dark` and `dark-root` mixins write it instead. The print block needed
`:root[data-theme]` alongside `:root` for the same reason: a pinned
theme is an attribute selector and would otherwise have outranked it and
printed the page on black paper.

Reading the stored value happens inline in <head>, above the stylesheet,
because a script that runs any later paints the wrong theme first and
corrects it. Nothing is written onto the button itself: which face the
icon wears, and which of its two labels is the accessible name, both
fall out of `data-theme` in CSS. That is not tidiness - hx-boost morphs
a freshly rendered masthead in on every navigation, and anything JS had
put on the element would be silently reverted. Two labels rather than
one so the button announces the action, not a state; display:none is
what keeps the other out of the accessible name.

Three places would otherwise have gone on following the OS and quietly
contradicted the choice. The theme-color metas keep their media queries
while the OS decides and are rewritten so exactly one matches once a
theme is pinned. The feed page honours the pin in both of its renderers,
the XSLT one and the feed.js standby, though it carries no switch of its
own - it is somewhere you pass through on the way to a reader. And the
hello world plate was declaring `color-scheme` on itself, which would
have left it the one thing on the page still reading the OS; dropping
the declaration lets it inherit the root's, pinned or not.

The spread is a view transition whose new snapshot is clipped to a
circle growing out of the button, over the old one held still
underneath - the same shape grafikart.fr uses. The pseudo-element rules
that hold those snapshots still are scoped to [data-theme-shift], and
that scope is the part worth keeping: hx-boost wraps every navigation in
a view transition too, and it is these same two pseudo-elements that
carry its cross-fade. Written bare, the rule would have flattened
page-to-page navigation into a hard cut. Anything without
startViewTransition, and anyone who has asked for less motion, changes
theme outright instead - a circle sweeping the whole screen is squarely
what that request is about.

Checked in Chromium against the minified build: no flash on load with a
theme pinned, the choice surviving a boosted navigation, six overlapping
transitions starting and finishing cleanly under a mashed button, the
metas flipping, both feed renderers dark on a light machine, and
reduced motion changing the theme with no transition at all.

The strings are in all five languages. Their wording in es, pt and ja is
mine and worth a second look.