Note: QGtkStyle has been removed from qt5-base 5.7.0 [1] and added to qt5-styleplugins AUR
If you have problems with Otter (or any other Qt app) not respecting Gtk style, Arch and Manjaro users have to install
qt5-styleplugins which will connect the necessary dots between Qt and Gtk automatically.
The same package exists on Debian as qt-style-plugins (https://packages.debian.org/sid/libs/qt5-style-plugins).
$ apt search qt5-style
Sorting... Done
Full Text Search... Done
qt5-style-plugins/testing,unstable 5.0.0+git16.g7aa4764-1 amd64
Qt 5 extra widget styles
You can set environment variables globally in xinitrc, see https://wiki.archlinux.org/index.php/Environment_variables#Graphical_applications
#~/.xinitrc
export QT_QPA_PLATFORMTHEME=gtk2
Or you can run a Qt app with a specific setting to try it out.
QT_QPA_PLATFORMTHEME=gtk2 otter-browser
Also, in FreeBSD, qt5-style-plugins (http://www.freshports.org/x11-themes/qt5-style-plugins/).