Hello, Since updating to Thunderbird 139, I’ve encountered a persistent issue with the dark theme in the reply composer. When replying to HTML messages, the text appears black on a dark background, making it nearly unreadable. This issue did not occur in Thunderbird 138 — nothing else changed on my system except the automatic update. Here’s what I’ve tried: - Verified that toolkit.legacyUserProfileCustomizations.stylesheets is set to true in about:config. - Created and edited a userContent.css file inside the active profile folder. - Used various CSS rules targeting messengercompose.xhtml to override text and background colors (e.g. forcing color: #ddd and background-color: #000). -Tried resetting composition preferences like msgcompose.background_color and msgcompose.text_color in prefs.js. - Set Thunderbird to use plain text composition, which works — but this is only a temporary workaround. - Ensured that no conflicting extensions (e.g. DeepDark) are enabled. -Confirmed the issue happens even with a clean profile. - Manually set the default text and background colors to white on black in the composition settings — but replies still render with black text on black background. The problem seems to be that Thunderbird 139 overrides the user-defined dark theme or does not apply CSS properly in replies. Is this a known regression? Is there any workaround to enforce dark-compatible colors in HTML reply composition without switching to plain text? Thanks for your help. Daniele