Something must have changed in GTK+, probably a long time ago, because
this code definitely used to work in the past.

Anyway, change gimp_ui_init() in libgimpui to use gtk_rc_parse()
instead of gtk_rc_add_default_file() which fixes the plug-in's initial
theme, and install a GFileMonitor on themerc which calls
gtk_rc_reparse_all() when themerc changes.