Bug: private background images incorrectly applied to mixed threads

When displaying a thread with mixed private and public mails, if the first mail is private then it appears that all the background has the diagonal private pink text [1]

However if the first e-mail is public [2], then private mails don't have the correct background.
Note: the thread does not display properly because of #296. Once this is fixed, the missing background can be noted.

[1] https://lists.apache.org/thread.html/0a32c28ab790abae8e6ba1788e02ab0aa52e2c76e66db4174d133354@%3Cprivate.ponymail.apache.org%3E

[2] https://lists.apache.org/thread.html/fa48c060348d7ce4e82c3accbeaaf4d858a04ad1ec86d761979e155a@%3Cdev.ponymail.apache.org%3E

Reproduce the mixed private/public thread behavior using the two linked examples, but first account for the rendering problem tracked in issue #296. Compare the cases where the first message is private versus public and confirm that each private mail receives the correct background; the payload names no source files or tests.

Written by the indexing model from the issue text.