The top-bar toggle went to light from anything dark, and back to plain
"dark" -- which quietly moved an ihasmail user onto a theme they had
never chosen, two clicks and no way to tell what had happened. It did
the same to "match system", which the toggle could not restore at all;
the comment above it conceded as much and sent people to Settings.
There is more than one way to be dark now, so the way back is
remembered: lastDarkTheme holds whichever non-light theme was last
chosen, and the toggle returns to that.
The remembering lives in update(), the single path every way of setting
a theme goes through -- the toggle, Appearance, an imported settings
file -- so a fourth way to choose one cannot forget to record it. Light
never overwrites it, since light is the side being toggled away from.
The button's label follows: "Switch to the ihasmail theme", "Switch to
your system theme", rather than claiming everything dark is "dark mode".
Confirmed in a browser, not only in tests: from a fresh profile the
round trip ihasmail -> light -> ihasmail returns to ihasmail, and
system -> light -> system returns to system, with the label naming the
destination each time.
jcoffey-dev
9689ac8aae