I guess that would be a bug in Welcome, as according to packages.ubuntu.com, arc-theme is not in the Xenial (16.04) repos (it was added in 16.10 IIRC). Numix is (main Gtk theme only, not the icons nor the blue variant).
My assumption would be that, as the list seems to be tailored manually, Welcome has been backported/updated with a list of packages made for more recent versions without checking that they indeed worked on 16.04.
You can still install the Numix theme manually:
sudo apt install numix-gtk-theme
If you want Arc theme on Xenial, you can add the repo of its designer: https://software.opensuse.org/download.html?project=home%3AHorst3180&package=arc-theme (don’t worry that it’s an OpenSuse address, there’s a repo for Ubuntu there).
If the option from Welcome only installs these two themes, it would then work after adding the repo (don’t forget to refresh your sources list). But as Welcome doesn’t list the name of this “extra themes” package, it’s not easy to check if that’s the case or if it wants to install more themes that may or may not be available on 16.04.
As Xenial is there for a few more years, it might be worth filing a bug on Launchpad so that the extra themes package for Xenial removes the dependancy on arc-theme (or directly ask @Wimpy ?).