Add a hook in elkarte_settings for
integrate_prepare_display_context
Example:
colorize_buddy|SUBSDIR/ColorizeBuddy.subs.php
<?php
function colorize_buddy(&$output, &$message)
{
global $user_info;
if (in_array($message['id_member'], $user_info['buddies']))
{
addInlineJavascript('
$( "#msg'. $message['id_msg'] . '" ).parent().css( "background-color", "#ccc" );
', true);
}
}