Cause:
It seems the Firefox set the following settings in about:config

This can occur if you’re using PolicyPak Application Settings Manager (and the Firefox 23 and later Pak) to REMOVE all AddOns.

Since THEMES are also AddOns.. Firefox then defaults downward to the DARK theme.

Resolution:

Choice 1: Use PolicyPak to force an Add-on which is a Theme that you want.

Choice 2: In our lab, problem goes away when we set the settings as shown in this screenshot:

lightweightThemes.SelectedThemeID = empty
lightweightThemes.usedThemes = true

To fix this on all your client machines please use Firefox Abou:Config PAKs and set the above value as instructed. Here is just a reference screenshot: