I was wondering if it is safe to delete Xfce themes and/or icon themes that I don't use to save disk space. Additionally, is there a apt or other package manager command to do this? Also, just in case this is important, I'm running Ubuntu 16.04. Thanks.
There is no reason at all not to remove those unused themes and icon sets.
You could remove the packages that installed them but those are all meta packages and do not install just one theme or icon set at a time. Therefore you need to check the content of the meta package to see what they install.
I have no idea what package managers with a gui are installed on Ubuntu of any version anymore.
The recommended way would be to rm the files from /user/share/themes or /icons using the cli.
The not recommended way would be to open your file browser as root and delete them that way.
If you are using Xubuntu you have an awful lot of Gnome things installed to make it work, like nautilus to run the desktop. If you installed the xfce-desktop to an ubuntu netinstall I am not sure what all is installed because it would be up to the Ubuntu folks what exactly was in that meta package. If it includes the xfce4-goodies you have a lot more theme and icons probably installed than a real xfce4 install would give you if unaltered from the xfce devs version.
Themes are usually not that large a file. Your space saving will be pretty minimal.
Icon sets are pretty large though so I would concentrate on those to start with.
The problem with removing them manually is that the package that installed them is still installed. If you moved the ones you use to a safe place and then found the packages from Ubuntu repos that installed them and removed those you could then put those icons back in /usr/share/icons.
That would work fine. Except that the package including the icons is probably part of a larger meta package and so removing that icon package will remove your, probably, entire desktop environment.
You can "break" meta packages with; aptitude keep-all
This simply changes the state of packages installed in a meta package from "auto" to "manual" allowing you to remove them individually.
This is not something that should really be used much. You can really screw up your package management system doing so.
It does work though.
Your best approach to a light system is to do a netinstall for only the basic system with no DE or any X11 packages at all and then build the system yourself using as few meta packages as possible. This is time consuming, requires a fairly large list of packages to install and is not real exciting to do. It is very satisfying to get that lighter, smaller installation.
It is also much easier to do on just about any distro that is not Ubuntu or Ubuntu based.
They repackage Debian packages for their repos. In doing so they create meta packages that use more than one Debian meta package meaning that a lot of desktop environments will, if removed, remove all X11 packages. While this makes it easier to install a desktop environment it also means it is much harder to remove package that you don't need at all.
If you are running actual Ubuntu with the xfce desktop added I would recommend re installing using just the Xubuntu 16.04 because you have a lot more packages related to Ubuntu and its Unity DE using space than any number of xfce icons or themes that you are not using.