I have now upgraded my OS to Ubuntu Mate 16.04 on RPi2.
Most of the buttons now have icons visible. Here and there an icon is missing.
Huge improvement, Great!!!!
stefan
I checked again and without any other manipulation all icons are no present on the buttons.
Also the new feature allowing to create profiles before launching and connecting the indi-server to devices works very well.
In order to repeat, I just upgraded to Ubuntu-Mate 16.04 and installed kstars-bleedingedge and it is all OK.
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.