Hi,
I am new to KDE and KDE Neon. I installed gnome-icon-theme, because 'meld' was not working at all and based on the error message it gave in the terminal I googled that solution. That fixed the folder icons on the bookmarks toolbar on my firefox, and probably some icons also in thunderbird (I don't remember exactly). However, I still cannot get the thunderbird's threaded conversation icon (small arrow / triangle) to show. I found a screenshot about that: http://write.flossmanuals.net/thunderbi ... readed.png . (I cannot put it to img tag due to max width limitation of 720 px.)
I rebooted to my Mint install (which also got some XFCE as well) and try to list which icon themes, etc. there are:
$ dpkg --get-selections | grep icon
adwaita-icon-theme install
adwaita-icon-theme-full install
gnome-icon-theme install
gnome-icon-theme-symbolic install
hicolor-icon-theme install
libtext-iconv-perl install
mint-x-icons install
mint-y-icons install
mintinstall-icons install

After installing adwaita-icon-theme and gnome-icon-theme-symbolic as well, the situation did not change (after restart of thunderbird). If I run thunderbird in the terminal, I don't see any relevant message about what extra icons it would require. hicolor-icon-theme was already installed, I believe. adwaita-icon-theme-full did not install:

Code: Select all

$ sudo apt install adwaita-icon-theme-full
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Starting pkgProblemResolver with broken count: 1
Starting 2 pkgProblemResolver with broken count: 1
Investigating (0) adwaita-icon-theme-full [ amd64 ] < none -> 3.18.0-2ubuntu3.1 > ( universe/gnome )
Broken adwaita-icon-theme-full:amd64 Depends on adwaita-icon-theme [ amd64 ] < 3.18.0+p16.04+git20170914.0754 > ( gnome ) (= 3.18.0-2ubuntu3.1)
  Considering adwaita-icon-theme:amd64 6 as a solution to adwaita-icon-theme-full:amd64 9999
Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 adwaita-icon-theme-full : Depends: adwaita-icon-theme (= 3.18.0-2ubuntu3.1) but 3.18.0+p16.04+git20170914.0754 is to be installed
E: Unable to correct problems, you have held broken packages.


Not sure how to solve it or whether I need adwaita-icon-theme-full in the first place.
Somewhere I read about oxygen-icon-theme, installed that as well, to no avail.
thunderbird version: 52.4.0.
I have the KDE Neon (non-LTS version with the latest updates).
Any ideas what package or setting thunderbird requires to show the threaded conversation icon?
Should not thunderbird, meld, firefox all depend on gnome-icon-theme (or maybe some meta package, not sure)? In my view it is a bug that they need it, still they do not depend on it in package management level. Would need to find out how and where to report bug for those...

Update:
A colleague recommended to install the "TT DeepDark" addon (https://services.addons.mozilla.org/en- ... -deepdark/) for Thunderbird which uses its own icon to expand/collapse the threaded conversation. I installed that and now I have + or - icons in a square in place of the previously missing icon.