Text in contact's tooltip is almost unreadable (theme error?)

[Vladimir Katilevskiy]

In 15.04 I have terrible looking contact's tooltips in Empathy with its content being almost unreadable (screenshot attached).

I compared tootlip section of the file /usr/share/themes/Ambiance|Radiance/gtk-3.0/gtk-widgets.css to the same file in Ubuntu 14.04 and found that original one (from Ubuntu 15.04) doesn't have the next:

.tooltip * {
  background-color: transparent;
}

Then I added this to gtk-widgets.css, switched themes forth and back and now I have nice tooltips like in previous versions of Ubuntu.

I am not sure if this is Empathy UI bug or Ubuntu's GTK (Ambiance and Radiance) themes' one. So let's start here.

System info:
Description: Ubuntu 15.04
Release: 15.04

empathy:
  Installed: 3.12.9-1ubuntu1.1
  Candidate: 3.12.9-1ubuntu1.1
  Version table:
 *** 3.12.9-1ubuntu1.1 0
        500 http://ru.archive.ubuntu.com/ubuntu/ vivid-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     3.12.9-1ubuntu1 0
        500 http://ru.archive.ubuntu.com/ubuntu/ vivid/main amd64 Packages

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.