Hmm, it seems to be an issue with the GTK2 style for Qt. Get Qt5ct (sudo apt install qt5ct), also see Can't use Qt5ct to change global application style to kvantum.
open qt5 settings from the menu, and change the style (to e.g. fusion). This will temporarily make qt5 apps use an old style. Afterwards, try again. If the issue is solved, I recommend you install Kvantum ( https://github.com/tsujan/Kvantum/tree/master/Kvantum) and choose one of those themes 