Not able to get a theme to apply on apps like Easytag. It's like there is no theme for them to apply. Same for the file dialog in Thunderbird.
See this image for details. What am I doing wrong? How can I fix this? See link below for link to view ...
https://www.dropbox.com/s/m52y9egd0jdd7yx/Screenshot_20170604_152716.png?dl=0

GTK and Qt apps use different themes. There are a couple of themes which have both GTK and Qt versions, including Breeze and Oxygen, and you can configure the Qt theme to just match the GTK theme, but there isn't a way to configure the GTK theme to just match the Qt theme.

To configure the GTK theme, go to System Settings -> Application Appearance -> GTK. (you may need to install the kde-gtk-config package (or whatever it's named in your distro) for this module to appear)