Perhaps it's a bit late in the day to be making mods for YaBB SE, but luiscamino asked
how to make the names of guest posters display differently, so I bashed through the code for him and now think I might as well produce a proper .mod file from what did!
This version improves on the original solution (for which I started by adapting an existing <font> element from Display.php) by adding an extra .guest class to your CSS so you can do whatever you like to the guest display names (including leaving them untouched if you change your mind). It also assumes that you know how to declare whatever style(s) you want to use, so please just ask if you don't!
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