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.
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?