You need to copy the dashboard/elements folder from the concrete/themes folder to your application/themes folder and edit line 66 of the application/themes/dashboard/elements/header.php file to suit your site.
Thanks a lot for your quick answer.
For some reason it does not allow to just replace the folder “elements” with the changed header.php file, you need to change the whole folder “dashboard”, then it works.
…unless I did something wrong. I hope it doesn’t bark somewhere else.