Dark mode is generally what most users find the most appealing while viewing emails. However, while viewing an email, sometimes dark mode does not carry over the intended message properly. Like as an example, when I get a barcode, the sender hopes the surrounding area of the barcode image is white in order for it to work.
In such cases, it should be possible quickly switch to light mode, instead of having to go to the settings. Maybe a view in light mode button could be added to the right top 3 dot menu that appears while viewing a mail.
Thunderbirdmobile