The default Roudcube favicon is… Ugly. Nothing wrong with the logo by itself, just not… as favicon…

Here’s how to change it:

First, Upload a new favicon (ico or small png recommended) to Roundcubes www-root (/var/www/html/roundcube for me)

Second, Verify that you can reach the previously uploaded favicon. Ex. example.com/favicon.{png,ico}

Third, Append to Roundcubes config/config.inc.php with the following

$config['skin_logo'] = [
   '[favicon]' => 'https://example.com/favicon.ico',
];

Good luck 💌