Comment 3 for bug 1750973

This issue (which did not occur with a fresh install of 16.04) appears to be related to the contents of file: /usr/share/themes/Ambiance/gtk-2.0/gtkrc

At line 346 (at present) there is the following:

    style "menu" = "dark" {

Changing it to:

    style "menu" {

... resolves this particular issue, however I'm not sure what side-effects that would have. In the original release the line was like that in the first place, so it would have been changed for a reason.

Screenshots of "before" and "after" that change can be seen here:

https://askubuntu.com/a/1014016/425017