I am having this same issue. gtk3-engines-oxygen was not installed on my machine. Installing, purging, and reinstalling this package did not solve this issue. I first noticed this same issue on August 8th, I manually installed updates via apt-get. The previous day it was all thunderbird updates, on the 6th it was libdbusmenu updates, firefox updates, passwd, and an xserver intel update.
I am having this same issue. gtk3-engines-oxygen was not installed on my machine. Installing, purging, and reinstalling this package did not solve this issue. I first noticed this same issue on August 8th, I manually installed updates via apt-get. The previous day it was all thunderbird updates, on the 6th it was libdbusmenu updates, firefox updates, passwd, and an xserver intel update.