Encontrei a solução depois de ler o link :
Qt5 decides the style to use based on what desktop environment is used. If it doesn't recognize the desktop environment, it falls back to a generic style. To force a specific style, you can set the QT_STYLE_OVERRIDE environment variable. Specifically, set it to gtk if you want to use the gtk theme. Qt5 applications also support the -style flag, which you can use to launch a Qt5 application with a specific style.
Então, adicionei essa linha ao meu $ HOME / .profile
export QT_STYLE_OVERRIDE=gtk