razorqt-qtplugin - a fork of lxqt-qtplugin which allows you to
change icon theme in Qt4 applications in RazorQt environtment

To compile it, invoke:
cmake ../ -DCMAKE_INSTALL_PREFIX=/usr
(in distributions <like Arch Linux> defaulting Qt5 instead of Qt4
add moreover -DQT_QMAKE_EXECUTABLE=qmake-qt4)
make install

Bear in mind it's a minimalistic workaround. To get it working do
following steps:
1) echo "your-theme-name" > ~/.razorqt_icontheme
"your-theme-name" could be for example " Faience-Azur
2) Add following environtment variable in RazorQt session settings:
QT_PLATFORM_PLUGIN=razorqt


Original lxqt-qtplugin readme:
libqtlxqt - LXDE-Qt system integration plugin for Qt.

With this plugin, all Qt-based programs can adopt settings of
LXDE-Qt, such as the icon theme.

To let Qt load the plugin, export the environment variable
QT_PLATFORM_PLUGIN=lxqt or DESKTOP_SESSION=lxqt.
However, setting DESKTOP_SESSION might affect the behaviors
of other tools such as xdg-tools. If this is not the desired
effect, use QT_PLATFORM_PLUGIN.