Please share what you mean by paragraph styles' and I'll try to assist. Thank you.

I'm trying to find this as well. I work with IT departments and frequently need to paste console output or code in emails. I use the "Preformat" paragraph style to change it to non-breaking lines and fixed-width font, but I also like to indent the text to distinguish it from what I'm writing. I would also like to be able to change it to a light gray background so it looks like a code block you'd see on a webpage.

I can easily add a style like this in LibreOffice Writer for example, but I can't find where to add or edit paragraph styles in Thunderbird.

The paragraph styles are on a drop-down on the left end of the bottom toolbar of the composition window, as well as on the menu Format > Paragraph. The styles available are Body Text, Paragraph, Heading 1-6, Address, and Preformat.

Libreoffice writes in RTF, which is not available to email clients. HTML is the tool. Creating indented blocks with different backgrounds or font colors is possible, but requires heavy focus on formatting instead of writing - and if the receiver uses plain text, all of the formatting will be lost.