Good catch.
It's due to a missing change in LLDir::hasSkin(). If you compile the viewer yourself, you can change the latter (in linden/indra/llfilesystem/lldir.cpp, lines 288+) to read:
Fixed for next release.
As a workaround, in the mean time, and since this is just a test issue, you can add an empty "skins/custom/colors_base.xml" file (i.e. in the old path) to "fool" the viewer.