Steps to reproduce:

preparation:

• fresh Windows-Sandbox on Windows 10
• open Edge, download and install the latest Firefox (currently version 95)
• enable dark theme in "about:addons"
• open a PDF file: it uses the dark theme (like expected)

bug:

• change "privacy.resistFingerprinting" to "true" in "about:config"
• switch back to the PDF file: the UI uses the white theme (which seems wrong; see attached screenshot)

Actual results:

The PDF viewer UI is using a white theme, despite everything else is set to dark mode.
When changing "privacy.resistFingerprinting" to "false", the PDF viewer UI is using the dark theme - like expected.

Bug discovered at https://www.reddit.com/r/FirefoxCSS/comments/rbvlcz/comment/hoyonet

Expected results:

I expect the PDF viewer UI to be in dark mode even with "privacy.resistFingerprinting" set to "true".

The Bugbug bot thinks this bug should belong to the 'Firefox::Theme' component, and is moving the bug to that component. Please revert this change in case you think the bot is wrong.

confirming with Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:97.0) Gecko/20100101 Firefox/97.0 Build: 20211217212339

I was able to reproduce this issue also on MacOS 11.6

I'm setting the flags where the issue is reproducible.

Voroti, could you use mozregression in order to find the Bug that introduced this?
The issue does not occur on ESR 91.

You can download the mozregression-tool from here:
https://mozilla.github.io/mozregression/

Thank you!

First time using the mozregression-tool, here is what it gave me:
https://phabricator.services.mozilla.com/D128611

Thanks!

The severity field is not set for this bug.
:bdahl, could you have a look please?

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