Creating a very simple warning message when users private message each other. In this case, I wanted a specific warning shown when sending a private message using a simple line of html and css code. Nothing fancy, nothing troublesome. This area can be used to your liking, ie; ads, rules, etc.1.
Open Themes/default/PersonalMessage.template.php and find:
<div class="pm-warning"><p>Please, do not send private messages to the author for support. Instead, reply to articles so others can also find the answer. <br /> Regards, The idesignSMF team.</p></div>';
Open your current theme's css file and at the very end add:
Replace hex colors to your liking as well as the text
Reference: About PersonalMessage.template.php scripting help