I am building a new computer with Ubuntu 16.04 LTS and just installed K3b 2.0.3 using the package manager. It seems like K3b is having trouble finding the icons for the buttons. The icons being displayed seem like they are some kind of fail-safe icons. They are very simplistic, monochrome looking icons that don't match what I'm seeing on my old computer (Ubuntu 12.04), nor what I've seen in screenshots of K3b 2.0.3 online. Another strange thing is that a few of the buttons will change to a normal looking icon when I hover over them with the mouse, but when I move the mouse away they revert back to the monochrome icons.
I am using Ubuntu-Mate as my desktop environment, but I have also tried Gnome, Gnome Classic, and Plasma to see if that would make any difference, but it didn't. So it seems like I am either missing a theme, or the files are being looked for in the wrong place. I haven't been able to find out which icon files it is trying to load, otherwise I could try making symlinks. I have also tried messing around in the Qt4 Settings GUI under System->Preferences->Look and Feel, but that didn't have any effect either. I am not sure what else to try.