[Igor Engelen]

Originally Posted by seanos

OK, I know this is a necro post but I’ve been researching this for a few Qt (4 & 5) programs so I’d appreciate finding this in a search.

What definitely works (at least in Ubuntu 16.04):

• Install Qt5 style plugins—sudo apt install qt5-style-plugins
• Copy /usr/share/applications/calibre.desktop to ~/.local/share/applications/calibre.desktop
• Edit the copy and change the Exec= line to Exec=env QT_STYLE_OVERRIDE='gtk' calibre
• Use this to launch calibre, i.e. either drag it to the launcher or make it executable and double-click it.

You should also be able to put QT_STYLE_OVERRIDE='gtk' into your /etc/environment file to do it for all Qt5 programs, but I haven‘t tested this.

More info:

• Details at the Arch Wiki
• StackExchange: Qt5 applications don't use gtk style on Liux Mint

Thanks man, you made my day. My Calibre is in black now like the rest of my apps. Finally I can rest