I've had this issue for a while now, since I thought I could fix it myself. Almost all my programs have lost their icon image, which is not fixable by applying a different icon theme unfortunately. Just installed the Reversal icon pack to test that. My settings are attached here, sorry for the german:
I am assuming this is due to some of the weird behaviour I have had for some days months ago. Gnome would just not load and instead show me an error screen. So I had to uninstall gnome entirely, then reinstall it and that magically fixed it, but my icons were gone.
Yikes. Something is missing. Is this Debian? You could swing by their forums or IRC and get sorted out fast. I'm sure there is a dpkg command that fixes this up. As for the icons, there is definitely a missing package related to adwaita or some lingering config file needs to be reset to defaults.
Pretty sure they meant you probably should've suggested a backup or given a heads-up of some sort before running the command you brought up, since one would presumably lose all settings previously stored in dconf and that's quite the extreme measure