Undefined variable 'zebra' in search.php

I received a screenshot from a user, which I attached to this issue. They are from opening the `search.php?search_id=unreadposts` page as logged in user.

It may be due to changed behaviour since PHP 7.3.0 that this did not surface before:

> compact() now issues an E_NOTICE level error if a given string refers to an unset variable. Formerly, such strings have been silently skipped.

This seems to be the issue here.