The former icon used fill="context-fill", which I believe doesn't work
except in Mozilla's own extensions. So I changed that one to
fill="black" and made a new one with fill="white".
In Firefox, this requires version 67 for support for
prefers-color-scheme media queries.
https://hacks.mozilla.org/2019/05/firefox-67-dark-mode-css-webrender/
To force Firefox into dark mode, set ui.systemUsesDarkTheme=1 (and
optionally browser.in-content.dark-mode=true, to put pages such as
about:addons into dark mode as well) in about:config. You can check if
it's working at https://bugzilla.mozilla.org/, which has its own
dark-mode styling. Note that this kind of dark mode is *independent* of
the "Dark" theme that can be selected in about:addons.
Chrome requires version 76 for prefers-color-scheme. You can force it by
running with the --force-dark-mode command-line option.
