Hold the theme switch at half light until it is pointed atUnverified
At full strength the icon sat level with the site's name across the row
and pulled the eye first, which is the wrong order for a control nobody
came for. Half-lit at rest and full on hover, the same treatment the
heading anchors already get, with focus-visible counted as pointing at
it so the keyboard is not left reading a dimmed control.
Touch screens keep it at full. There "at rest" is the only state there
is - nothing can hover - so a dimmed switch would simply stay dimmed,
and this is the one way to change the theme.
Worth recording rather than discovering later: at 0.5 the icon is about
2:1 against the paper in both themes, where WCAG 1.4.11 asks 3:1 of a
control that is not text. It clears that from roughly 0.75 upwards, and
reaches 4.9:1 at full. Hovered it is the accent colour and well past it
either way.