I run some KDE applications on Ubuntu 16.04 Unity, such as KDE Connect.
I seem to be missing some packages that provide button icons:
Does anyone know which packages to install to have these buttons being shown correctly?
As JonasCz pointed out, you need to install
oxygen-icon-theme. But it's not enough to have installed, you also have to use it.
One way of doing this is to install Unity Tweak Tool:
sudo apt install unity-tweak-tool
Run it, go to Appearance / Icons and choose "Oxygen" instead of "Ubuntu-mono-dark" or whatever your current theme is. Then open the KDE Connect window again and it should have the icons.
If you don't like the Oxygen theme you can try another one. KDE Connect is not tied to Oxygen. What you need is a theme with specific icon names. For example, the button from your first screenshot needs an icon named
edit-rename (see the reference in the source code):
You can search on your computer for icon themes containing such an icon:
find /usr/share/icons -name 'edit-rename*'
You could also search for available packages containing the icon:
apt-file find edit-rename.png
Or maybe you could even modify your current theme and create links with the desired names to the existing icons (I haven't tried this).