There is a list of all squirrelmail themes, but it seems not to be complete and all up-to-date. Also it is not linked at in the docs.
Looking through all themes in preferences in squirrelmail and reload the page is a very time consuming procedure. So there should be a complete collection with scereenshots of all themes. and a link to it next to the selectbox.
For example these are missing in the squirrelmail themes page:
discussion: http://webapps.stackexchange.com/questions/60123/select-the-fitting-squirrelmail-theme
It seems like all new themes are missing:
It would be helpful to add the numbers used in the config file to the selectbox and overview as well