I can't reproduce using Thunderbird 60.9.0 on Linux Mint 19.2 or Ubuntu 18.04.3 LTS with the pref set to true, and the built-in Dark theme, or Light and Default themes. I also don't have the extensive customization the reporter has in the screenshot. Maybe that is causing a problem?
Bug 1593578 Comment 2 Edit History
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I can't reproduce using Thunderbird 60.9.0 on Linux Mint 19.2 or Ubuntu 18.04.3 LTS with the pref set to true, and the built-in Dark theme, or Light and Default themes. I also don't have the extensive customization the reporter has in the screenshot. I also don't have the resolution their monitor displays. Maybe that is causing a problem?