Hi

Don't know if this can be done for external themes, but at least in the included ones it might be worth adding virtualboxvm under the opaque apps list - since the machine and the vb window does not share a name.

More info in this manjaro forum topic
https://forum.manjaro.org/t/kde-virtualbox-machine-visual-problem-with-kvantum-theme-enabled/100723/14