To make the best out of flatpak, it's best to think of it as a new sandboxed distribution that lives on your host system. E.g You could check if a specific theme is installable as a flatpak application.

Flatpaks are not perfect and I have my gripes with flatpak but the only alternative is Ubuntu's snaps.

Ubuntu's Snaps depend heavily on AppArmor which is not present in many of the other top-tier distros which use SELinux, so it's not an alternative.