*8913* Prevent fatal error when Custom Theme plugin is used in Site Context

http://pkp.sfu.ca/support/forum/viewtopic.php?f=8&t=12867&p=49274#p49274
http://pkp.sfu.ca/bugzilla/show_bug.cgi?id=8913

There's a constant defined for this purpose -- CONTEXT_ID_NONE. It's actually 0, not null.