I've installed steam from repo, then opened it, it did the usual update. After the update there was a blank login screen. Closed it, then reopened it but it is still blank.
As fa as I see, the login screen actually works, but there are no buttons, no text at all.
If I click somewhere at the bottom, the login user login screen opens but when you enter your username and password, there is another dialog, Steam Guard, which also has blank areas.
Kubuntu 15.10 + backports ppa and I have nvidia gtx 850 m and nvidia propiriety drivers (352.63) installed.