Adjust Greeter's CSS for Stretch

I did not check if the same happens on Jessie, but on Stretch the Journal says:

(tails-greeter.py:3017): Gtk-WARNING **: Theme parsing error: greeter.css:1:30: The :insensitive pseudo-class is deprecated. Use :disabled instead.

Maybe we can have something that works both on Jessie and Stretch, and is not deprecated?

Original created by @intrigeri on 11704 (Redmine)

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