I've recently installed the canonical version of Krusader (2.4.0-beta3) on Ubuntu 16.04, but whatever I do I can't get it to use the system icons.
Krusader seems to use "Breeze" (I think ... but I'm not sure ...), while I'm using "Ubuntu-mono-dark". I've also tried other icon themes but Krusader looks the same.

I'm aware of a similar problem when trying to run Krusader in root mode on Kubuntu (https://forum.kde.org/viewtopic.php?f=225&t=140091&hilit=krusader+icon), but it seems to me it's unrelated.

Can anyone help me with this issue?

You probably won't be able to use a different icon theme other than Breeze - consistently with all icons - because only Breeze includes all icons Krusader needs.

Also version 2.4.0 is hopelessly outdated. A fallback option for the icon theme was recently added with 2.7.0, see https://bugs.kde.org/show_bug.cgi?id=372966. There you can also find information about changing the icon theme used by Krusader if you still want to use the Ubuntu theme.

Thanks for the reply.

Actually, I've initially installed version 2.7.0, but I got the same behaviour described above. I then downgraded to the proprietary version, because I was hoping that would solve the problem easily (I've used that version before and it worked just fine - apart for the missing icons issue).
I just don't like the breeze-theme (I'm used to something else..), so I don't mind a few missing icons.

Anyway, tomorrow I'll try some of the methods suggested in the link you posted. Hope they will work!

So, I finally managed to solve the problem! But it seems to me none of the suggested fixes helped.
I believe what really resolved the icons issue was moving to the git repository version of Krusader (2.8.0).

For the sake of future reference, here's what I did:
Following the answer in https://askubuntu.com/questions/779796/how-to-integrate-kdenlive-into-the-ubuntu-desktop-design
1) To have menus in the kde application window as well, I run

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sudo apt-get install appmenu-qt appmenu-qt5

2) I installed the qt5 configuration tool

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sudo add-apt-repository -y ppa:nilarimogard/webupd8
sudo apt update
sudo apt install qt5ct

3) run qt5ct and
a) Set the Style to GTK+
b) In the Icon Theme tab, I selected the icon theme I wanted.
4) Edit ~/.profile and add at the end of the file

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export QT_QPA_PLATFORMTHEME="qt5ct"

5) Also added to the end of the /etc/environment file

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QT_QPA_PLATFORMTHEME="qt5ct"

6) Logged out and logged in again.

This didn't resolve my problem! (but I don't know if it didn't help at all).

Finally, I removed the proprietary version of Krudader, cloned the git version, followed the instructions in the INSTALL file, and compiled krusader.
Now everything seems to work well!