In environments besides KDE, Qt apps stick out like a sore thumb as they don’t respect your theme. I spent some time trying to fix this with Kvantum but it didn’t seem to work. After what I declared to be a long enough time putting up with it, today I finally found this page that explains qt5ct is the solution.
To summarize, first get Kvantum and qt5ct. For Arch-based distros like mine:
sudo pacman -S kvantum-qt5 qt5ct
Then you can apply a theme in Kvantum Manager. It’s worth noting that you can likely download a theme that matches what you’re using for GTK if there isn’t one built-in.
It’s also worth noting that this doesn’t work yet. Open up qt5ct (itself a Qt app; note the style of the window) and change the “Style” dropdown to “kvantum” and hit “Apply.” Wait a moment and you’ll see the qt5ct window change before your very eyes! Now you can open Qt apps without blinding yourself.