[Bug 1958972] [NEW] Thunderbird 91 does not honor the dark theme

[Giuseppe D'Angelo]

Public bug reported:

Description:	Ubuntu 20.04.3 LTS
Release:	20.04

thunderbird:
  Installed: 1:91.5.0+build1-0ubuntu0.20.04.1
  Candidate: 1:91.5.0+build1-0ubuntu0.20.04.1


After the upgrade to Thunderbird 91, the dark theme settings are not
honored in several places:

* when reading an email message (either in the bottom right corner panel, or when opening a message in its own tab/window)
* in the preferences

These areas are "black on white" (light theme) rather than following the
system settings (dark theme). The rest of the application is using dark
theme.

Going into Preferences -> Language & Appearance -> Colors has "Use
system colors" selected by default. Also, it has "Override the colors
specified by the content with my selection above" set to "Always".

ANY SELECTION OF THESE TWO PARAMETERS IS BROKEN:

* As I said, choosing "Always" + "Use system colors" means that
Preferences + Emails are in light theme.

* "Never" + "Use system colors", then the Preferences page uses the
correct dark theme. The emails still appear in a light theme.

* "Never" + UNSET "Use system colors", and force a white on black (dark)
theme, then the preferences are in the correct dark theme (not the one
forced, but the system one). The emails will appear white on black (the
one forced), which is still wrong, as the system dark theme isn't black.
However, if an email forces black text (e.g. HTML body), then it becomes
unreadable (black text over black background).

* "Always" + UNSET "Use system colors", then Preferences are in dark
theme (but the one I forced, not the system one), email bodies are white
on black (again, the one I forced), but any color information from the
email is overridden. An HTML email with the usual "my replies are in
red" won't have any red text.


This is a clear regression from the earlier version of Thunderbird where all of this worked perfectly.

** Affects: thunderbird (Ubuntu)
     Importance: Undecided
         Status: New

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Title:
  Thunderbird 91 does not honor the dark theme

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