My efforts over the last few days to make the new thunderbird 78 an acceptable platform are available here.
formatting link
You need to unzip that - it contains the directory - 'chrome' in your thunderbird profile directory.Then enable thunderbird to use it by setting the config variable as detailed here.
formatting link
essentially setting the config variable:toolkit.legacyUserProfileCustomizations.stylesheets
to 'true'
If you dont like it, edit the files, delete them or set
toolkit.legacyUserProfileCustomizations.stylesheets
to false again.
I use Liberation Serif as a basic font - if you don't have it use something else. Offered entirely unsupported.