How to use dark mode for the entire Thunderbird interface?

[pinemach]

How can I achieve an actual dark mode with Thunderbird? I've got most of the interface showing a dark mode, but the frame around email content in particular evades me.

Here's an image showing what I mean: https://i.imgur.com/WBP9yGE.png

Thanks!

[WaltS48]

What version number of Thunderbird on what operating system?

The current version 78.9.1 has three built-in themes, Default, Dark and Light,

I use a dark operating system theme with the Default Thunderbird theme on Linux, the default OS theme and the Dark Thunderbird theme on Windows 10.

Sorry, I can't help with CSS code,

[pinemach]

I'm using Thunderbird 78.6.1 (64-bit) on Windows 7 currently. The provided screenshot is with the dark theme selected. I have no problem writing my own CSS, though I would need documentation on selectors and how to load custom CSS in the first place.