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GNOME is a desktop environment built upon the GTK widget toolkit. However, VLC and VirtualBox are based upon the Qt toolkit. As such, you have to select a Qt theme for those applications.
Apart from the regular Qt themes ─ e.g. Breeze ─ you can use the Kvantum Manager to apply an SVG-based theme. The Kvantum Manager itself already comes with a number of
kvantum application themes, but you can also get new ones from store.kde.org.
The way to do this is to download one of those themes, which usually come in the form of a
.tar.gz package. You then extract that package somewhere in your
$HOME, and you point the Kvantum Manager at the correct folder with the unpacked files, and you load the theme.