PSA: Built-in Light / Dark theme set-up change

[Emilio Cobos Álvarez]

Assuming things stick, the manifests for our built-in light / dark
themes will be mostly empty as of bug 1993056.

Instead, these themes will be implemented by doing a couple tweaks to
our system theme:

* Disabling native colors if applicable (Windows and macOS already do
that by default). This is done via the (-moz-native-theme) media query,
which replaces -moz-pref("browser.theme.native-theme").

* Forcing the color-scheme to be the one of the relevant theme.

This makes it easier to make theme changes to our front-end, and avoids
accidental divergences between the default macOS / windows themes and
our light / dark themes.

Let me know if you have any concerns with this.

Thanks,

-- Emilio