I have just made a clean install of Ubuntu Mate 16.04 64Bit and kstars-bleeding on a laptop and icons did not appear.
Made a sudo apt-get install oxygen-icon-theme and also solved the problem.
Be careful with this. I tried this yesterday because he reclaimed on dependencies and it completely broke my system (xubuntu). Conflicts between qt5 versions can happen.
Yes that may happen if you have more than one version installed, I had no problems either with the RPi or the 2 PC's I have installed.
It didn't ask me for dependencies, maybe because I had them already installed.
Anyway thanks for the warning, I should mention, first try the last post:
sudo apt-get install oxygen-icon-theme
that fixed the problem too and it's probably safer.
Ok, but installing oxygen icons fixed my missing icons issues, as it seems to do for others here also. Maybe it is something different which oxygen icons package depend upon that is missing and being installed in the process?
This is still an issue on Ubuntu 19.10, using kstars 3.3.6, and as this is the top result when searching for this issue I thought I should reply with my results.
Installing oxygen-icon-theme did not fix the issue for me, but breeze-icon-theme did. This should probably be set as a dependency for the kstars package. I have no idea if this is the right place to point this out or if that should be done somewhere else.
Update: Current install on Orange Pi with Ubuntu Focal (Server with Xubuntu minimal) February 2023. Installing 'breeze-icon-theme' also solved the problem.