you can modify the line with $message=... it's simply bbcoding involved. Currently its
First color is set to silver for the username. 2nd Color is set to white for the rest of the thread. You can remove the bbcoding and it will default to the normal color. I apologize my theme is set to dark so I left it for a dark theme, anyone with a bright theme won't be able to see squat lol. I'll post a version stripped of bbcode in a bit.
PHP Code:
$message = '[color=silver][b]' . $username . '[/b][/color][b][/b][color=white] replied to the thread[/color] ' . $titleprefix . unhtmlspecialchars($this->info['thread']['title']) . $titlesuffix;