After installing Kstars and friends on my Mint 19 desktop from the launchpad PPA, I discovered that none of the buttons in the application had icons. Some were blank, whilst others had text overlaying them. I only actually realised this wasn’t intentional when watching videos about how to use Kstars.
Searching for possible solutions I found all sorts of possibilities mentioned, but what actually fixed things for me in the end was just this:
$ sudo apt-get install breeze-icon-theme
A restart of Kstars then showed all the icons, which makes it somewhat easier to use as the toolbars can contain many buttons.