First attempt to install via Flathub on Mint produced error about icon theme
I tried to install gnome-boxes via the download button on https://wiki.gnome.org/Apps/Boxes. I had read the prerequiste document for my distribution - Mint - and that document said there was nothing I needed to do. The download button opened Mint's Software Centre, together with an install button therein. I clicked the button and soon after received an error - that I did not manage to screenshot and which I cannot find in my logs about my icon theme, which is Paper (https://itsfoss.com/install-paper-theme-linux/). The installation aborted. The second time I clicked 'install' within the aforementioned Software Manager, the installation proceeded.
This might well be a problem with Mint's Software Manager, but the Mint dev's are tired of me submitting bug reports and the problem might lie with Boxes itself.