Comment 6 for bug 1224438

The root cause is the use of transparency in the Lubuntu-default theme. Can be fixed by editing:

/usr/share/themes/Lubuntu-default /gtk-3.0/gtk-widgets-toolbar.css

And replacing '@transparent' with '@bg_color'.

Deleting the file altogether also seems to work with no apparent problems.

Lubuntu Trusty Beta 2 no longer has this file and the buttons in evince have been removed in favor of a more traditional menu bar.