• TB 78.0b2 (64-bit) with default or dark theme
• W Pro b19041.329 with dark OS theme

78 b1 introduced dark message body background, which is lovely, with the exception that the text colour does not get inverted (or sufficiently contrasted) and with black text colour on dark background makes message composition a bit difficult.

I do not understand why this is supposed to be a duplicate. The cited bug is a different

Component: Message Reader UI

whilst this bug been reported for

Component: Message Compose Window

The other bug does not exhibit on my node and neither is in the other bug thread any mentioning of an issue with the Composer Window.

as suggested in bug#1639249 installed from https://archive.mozilla.org/pub/thunderbird/candidates/79.0b3-candidates/build1/win64/en-GB/

• OS W10 x64 Pro 2004 b19041.388
• TB 79.0b3 x64 (Build ID 20200725020053)

but the issue is not remedied.

The font colour is being set by (inherited from) chrome://messenger/skin/messengercompose/messengercompose.css L71

color: -moz-dialogtext;

and applied @media rules

@media (-moz-os-version: windows-win10) and (-moz-windows-default-theme) { #menubar-items > #mail-menubar > menu { -moz-appearance: none; border-style: none; }

@media (-moz-windows-default-theme) { @media (-moz-os-version: windows-win8), (-moz-os-version: windows-win10) { #compose-toolbox:not(:-moz-lwtheme) { --chrome-content-separator-color: #c2c2c2; } } }

@media (-moz-windows-default-theme) { menulist { -moz-appearance: none; margin: 1px 4px; padding: 1px 2px; } }

Well, as https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme#Examples states

If you have set privacy.resistFingerprinting to true, prefers-color-scheme preference is overridden to light.

Which in fact been the case, reverting to privacy.resistFingerprinting;false resolves the issue.