Technically virtualbox
uses Qt5 :
virtualbox eopkg4-bin info virtualbox-common
virtualbox-common package is not installed
Package found in Solus repository:
Name : virtualbox-common, version: 7.0.14, release: 314
Summary : Common components for VirtualBox host packages
Description : VirtualBox is a free powerful open source solution for running other x86 and x86_64 operating systems virtually on your computer.
Installation Guide : https://help.getsol.us/docs/user/software/virtualization/virtualbox/
Licenses : GPL-2.0-only
Component : virt
Dependencies : libtpms libxcursor libxinerama libxmu qt5-tools qt5-x11 extras sdl1
Distribution : Solus, Dist. Release: 1
Architecture : x86_64, Installed Size: 121.25 MB, Package Size: 36.43 MB
Reverse Dependencies: virtualbox-devel virtualbox-current virtualbox
Whether it looks good on Plasma, I don't know. I myself only use virt-manager
in any desktop environment.