Did you get anywhere with this? I have just updated to 4.9.0 on the Chromebooks in Kiosk mode. Also wondering it i can change the default background.
They said a fix is in the next cleint release, by the end of the year. Searched for and wide! I think you can change the background by messing with the Web Client files on your Connection Server.
We are about to try this. Let me know if it works, Are you messsing with the JSON configuration files at all? Wondering if I can skip the SSL trust.
You can't change the background for the app just the HTML login.