Oh, in fact it's the same with other Qt5 apps.
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Qt5 apps don’t pick up the GTK+ style anymore
Status in qtbase-opensource-src package in Ubuntu:
Since I upgraded to Qt 5.7.1, Qt5 apps stopped picking up the Ubuntu
(GTK+, I presume) theme, and look very Windowsy. I have qt5-gtk-
platformtheme installed, and I tried forcing it by setting
QT_QPA_PLATFORMTHEME, but to no avail.
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