(Not to be confused with the similarly named plasma-browser-integration) plasma-integration, quoting GitHub[0], "[i]ntegrates Qt applications with the KDE workspace by providing a QPlatformTheme", or in other words, makes Qt applications not provided by KDE correctly use the theme set by KDE.
I ran into this issue by installing transmission-qt and noticing that it was not using the Breeze theme that I had set in KDE's settings. Installing the plasma-integration package fixes this, but I feel that this is something that should be handled by adding plasma-desktop-service-type to my list of system services. Thoughts on that? P.S. I also think that might be a good idea for the kded package, but I know no longer recall why I added that to my list of packages to install. [0] https://github.com/KDE/plasma-integration
