The background color of received plain text messages can be changed in the Colors window in Options/General/Language & Appearance (see picture). To modify the colors for html messages in Dark Mode, there is Darko.

The background color of received plain text messages can be changed in the Colors window in Options/General/Language & Appearance (see picture). To modify the colors for html messages in Dark Mode, there is Darko.

Thanks, but that answers neither of my questions. The background color I asked about was just an example, I tried to make that clear.

Regards, Gertjan.

The reading pane (basically the same pane as used when you open in a tab) is like a web browser window going to draw the background colour from the HTML email it shows unless that is not specified.

To determine what the pane is called in the CSS, use the developer tools. (ctrl+Shift +I)

I wish I hadn't mentioned this background color... ;-) I had already looked at the developer tools. The reading pane is an element called browser, with id messagepane. The tools also allow adding a style sheet, which I did for experimentation purposes. I added the following style there, hoping to see some change:

 #messagepane body div.moz-text-plain { font-weight: bold !important; }

However, nothing changes. (I tried various selectors here, with various message types.) If I do the exact same thing in Firefox, with an appropriate selector, this works as expected.

Regards, Gertjan.

re : I have not been able to make any styling changes to the message (pre)view pane.

What are the actual changes are you trying to enforce in the 'Message Pane' where you read incoming emails ? Are you only trying to enforce a bold font in Message Pane ?

It would seem you are trying to influence much more than the options as mentioned by sfhowes.

userContent.css

body {font-weight: bold; !important;}

This will use bold font in Message Pane , but it also set bold font in 'Write' window as well.

or more specific .....

    .moz-text-plain,
    .moz-text-plain    pre,
    .moz-text-flowed,
    body { font-weight: bold !important; }

Message Pane fonts for received messages are usually set in the Advanced section of Options/General/Language & Appearance. An example of userChrome for the Header Pane (top of the Message Pane) is:

#msgHeaderViewDeck {
  background-color: lightblue !important;
  border-bottom: 0 !important;
}

#compactHeadersBox {
  background-color: lightblue !important;
  border-bottom: 0 !important;
}

The second part is only needed if you use the Compact Headers add-on.

There are further examples here.

Toad-Hall said [...] Thanks, that got me going. (Note that there is a syntax error in your first example: there should be no semicolon between bold and !important. As it turns out, the !important isn't needed here.)

I had a stupid error in my userContent.css; it started with an invalid quote (/ *, note the space), which rendered the first selector invalid.

On top of that, anything I tried in the inspector failed. I found out that, to test out stuff in the message pane, you need to click the three-pane icon in the top right corner. It opens a dropdown; the (for me) third item is the message I'm currently viewing. Selecting that, then (on the style editor tab) clicking the +-icon to add a stylesheet, allows me to enter CSS that is appied to the current message immediately. What I enter there works from userContent.css as well.

What I'm currently doing is mostly for newsgroup messages; I wanted to set a background color for quotes. Something like this works for me:

blockquote[type="cite"], .gmail_quote {
  color: #00c;
  background-color: #eee;
  border-left: none;
  padding-left: none;
}

(I don't know yet if this will make editing a reply less readable.) But mostly I just wanted to play around.

Thanks again, regards, Gertjan.

An example of userChrome for the Header Pane (top of the Message Pane) is [...] [...] There are further examples here.

Thanks. The link is informative, and I had not found it. But this is exclusively for the "chrome" part of Thunderbird; the first selector on that link selects everything, and the comment above it explicitly states that the content (viewing pane) is not affected. I did try some stuff in userChrome.css before asking here; this, for example, has an interesting effect: