I installed calibre from their site, not the packaged version, because it lags behind. It's working fine, but is using a different theme (plastic I guess) instead of my current theme (numix). Is there a way to fix that?
I would like to write more, but it's already explained here: How can I theme my Qt app to resemble one written in Gtk?
This little tool will allow you to switch to your selected gtk+ theme.