Usually, if you have all the appropriate packages installed, running a GTK+ app in KDE/LXQt, or a Qt app in GNOME/XFCE/Cinnamon will look normal. The widgets/th

Usually, if you have all the appropriate packages installed, running a GTK+ app in KDE/LXQt, or a Qt app in GNOME/XFCE/Cinnamon will look normal. The widgets/theme will not normally look identical to the WM's theme and widgets (Unless you're using a cross-widget theme such as Oxygen-gtk+), but you won't be seeing any real issues or loss of functionality. Should an application (let's say, Amarok) require chunks of KDE to install on your nice GNOME desktop, your package manager will install those required packages along with the application, so that when it starts up, there are no missing libraries or errors. And those libraries will be loaded (in the case of Qt4, 50MB of libs) into memory for as long as the application is running. Now, should you be running a headless install without all the appropriate lipstick packages already installed (some of us like running X apps over SSH sessions), you may see some really ugly naked widgets. Qt and GTK+ without any theming engine installed will not win any beauty pagents, but you'll be able to use the application, even without icons. And, should you wish to place some lipstick back on that pig, you just need to install a package or two to pretty everything back up. TL;DR - Stuff should work fine between DE's, if it doesn't, then your distro is broken or frankenstein'd.