In "app store" environments like Android, customized UI themes are a fairly popular category of user-installable component. Free desktops have two main types of theme:

• icon themes, like the ones in https://packages.debian.org/sid/all/numix-icon-theme/filelist (multiple themes in one package here)
• GUI toolkit and window manager themes, like the ones in https://packages.debian.org/sid/all/arc-theme/filelist (multiple themes in one package again)

plus some more minor ones (Debian has a couple of sound themes and a few cursor themes).

Is AppStream an appropriate way to describe these theme packages?

• Themes should maybe have their own type, unless they are considered to be an addon to something (what?)
• Icon themes should maybe be their own top-level <provides>, like <provides><icon-theme>Numix-Light</></>?
• The various other kinds of theme often go together as a group to give multiple toolkits and packages a coherent look, so maybe <provides><theme types="cinnamon gnome-shell gtk-2.0 gtk-3.0 metacity-1 plank unity xfwm4">Arc</></> or something?