So when you install KDE on Linux Mint (MATE edition) by going “sudo apt-get install kde-full” it works well except GTK apps like firefox, openoffice, xchat and filezilla will look crap.
sudo apt-get install kde-config-gtk kde-config-gtk-style qtcurve
Then go to Menu > System Settings > Application Appearance > GTK Appearance and select the “qtcurve” theme.
Another helpful hint: Go to System Settings > Workspace Appearance > Window Decorations and choose “qtcurve” or something else instead of Oxygen. Oxygen doesn’t allow you to have different coloured titlebars depending on active window state, making it rather tricky to identify the currently active window (especially with focus follows mouse on).