Você já tentou Qt5Ct ?
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Unlike Qt4, Qt5 doesn't come with a configuration tool to allow setting the icon theme, fonts or the style and instead, it tries to use the settings from the running desktop environment.
To force the Qt5 style or icon theme, you can use an application called Qt5ct (Qt5 Configuration Tool). Besides the style and icons, Qt5ct can also be used to change various other Qt5 settings, such as fonts, add custom style sheets and tweak other interface settings such as the double click interval, enable icons in menus and dialog buttons and more.
Aqui está o pacote download