A lot of my Qt apps (e.g., qpdfview, VLC) recently stopped respecting my GTK theme. I did some checking, and it turns out they are all Qt5 apps. Google led me to the QT5 apps don't obey GTK theme settings post at the Manjaro Linux forum, which has some interesting things to say (generally applicable to all distributions).
What I eventually did was add:
# Workaround for Qt5 app theming. export QT_STYLE_OVERRIDE=gtk
to the end of my my .profile, logout and log back in. Works now.