Bug fixes, covering 1.2.0 and 1.2.1.

• Turning "stubborn sites" back off no longer switches Nocturne off everywhere.
  Unticking it gave back the permission the add-on's own content script runs on,
  so from the next page load there was no dark mode on any site, nothing in the
  interface to explain it, and no way to put it back except by reinstalling.
• More sites get their own dark theme instead of a generated one: those whose
  stylesheets use the older "only screen and (prefers-color-scheme: dark)" form,
  and those where an embedded frame was deciding for the whole page.
• Settings stay where you put them with more than one tab open, and the popup no
  longer describes a site the way it was before your click.
• Colours written in newer CSS syntax are themed properly instead of coming out
  grey.
• Stubborn sites: no white flash when you change a setting, nothing left welded
  onto the page after an in-page link, and content that appears after the page
  loads is themed like the rest of it.

Bug fix release.

Per-site settings work now. Nocturne could not tell which site you were on, so the site name, the on/off switch and the per-site sliders all did nothing, the toolbar icon never showed it was on, and the per-site keyboard shortcut did nothing either.

Also fixed:

• Pages no longer drift towards flat grey a second after they load.
• A site you switch off can be switched back on.
• "Site theme only" no longer recolours a site you have visited before.
• A change in the popup no longer undoes one made in the settings page.
• With "Stubborn sites" on, switching Nocturne off now really switches it off.
• The schedule time fields are labelled for screen readers.