Support themes add-ons

[daozhihun]

Why/User Benefit/User Problem

Some people may prefer to install the themes that supported on old Firefox for Android.

What/Requirements

Looks like only the address bar needs to hange.

Acceptance Criteria (how do I know when I’m done?)

Support themes for the older version of Firefox for Android. After active a theme, the address bar changes accordingly.

┆Issue is synchronized with this Jira Task

[buttercookie42]

Looks like only the address bar needs to hange.

Together with the phone's status bar (using the the theme's frame/formerly accentcolor property, respectively auto-generating our own matching color if that property wasn't specified).

And I've just remembered that the theme image was also used as a (muted) background for the tabs tray, too.

[boek]

Moved to bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1813946

Change performed by the Move to Bugzilla add-on.