In the article "New! JSON themes in Kopano WebApp" by the 16.07.2018 Martjin Alberts wrote about the possibility to make your own themes. When I read the instructions I thought cool but how should it go really?

I don't write again what he wrote's, I will only describe the steps which you will to change to your own themee

Under

/usr/share/kopano-webapp/plugins/
you make your own directory which the files and other under-directories are will be

The theme-call you make in

/etc/kopano/webapp/config.php
by the change from
'define("THEME", '')
to
'define("THEME", '{YourDirectoryWhatYouMaked}')

When you had restarted your WebServer and call your Kopano WebApp Site you should see your changes

So you have your own Company or Club-Design which you want

Theme from Kopano Own theme