Aqui: link
Talvez você precise disso: "Para ativar o suporte, defina QT_QPA_PLATFORMTHEME = appmenu-qt5 em seu ambiente."
Você pode fazer isso editando o arquivo ~/.profile
e adicionando a linha export QT_QPA_PLATFORMTHEME=qt5ct
no final do arquivo, como no exemplo a seguir:
# ~/.profile: executed by the command interpreter for login shells.
# This file is not read by bash(1), if ~/.bash_profile or ~/.bash_login
# exists.
# see /usr/share/doc/bash/examples/startup-files for examples.
# the files are located in the bash-doc package.
# the default umask is set in /etc/profile; for setting the umask
# for ssh logins, install and configure the libpam-umask package.
#umask 022
# if running bash
if [ -n "$BASH_VERSION" ]; then
# include .bashrc if it exists
if [ -f "$HOME/.bashrc" ]; then
. "$HOME/.bashrc"
fi
fi
# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
PATH="$HOME/bin:$PATH"
fi
export QT_QPA_PLATFORMTHEME=qt5ct