Kerio changed some things in the latest major version beyond the branding of Mailserver to Connect. The way one could modify/personalize the login screen for users also changed. It used to be that you could just edit the .css file in a directory and insert a logo, etc. and it is similiar to the way it was done in pre-7, pre-Kerio Connect, but the files are different.
I am running Kerio Connect on OS X currently, but Linux installs should be the same path.
First files you want to backup and then mess with is located here:
These are the main css files for that login page. You will be able to change colors and backgrounds, etc. here. I always copy the default one off to the side as well as the customized one to have around after updates reset them back to defaults.
The other file is right next to the other:
This file controls the page title, and design content of the page. Always good to change some stuff so it isn’t so generic.
Good luck and it takes some time to play with it to get it the way you want, but backup and always keep something off to the side for quickly being able to swap it back in after Kerio updates are played. I always create a directory in the vicinity that houses optimized logos, etc. and backup files just in case the get hosed during testing.
We took the bubble images and turned them black since we didn’t want to mess too much with the design knowing that Kerio will stick to this for a while. I also did sym-links (ln -s) to the file names so those symlinks will get hit with the same filenames on upgrades but not the actual image files.