The address
   /themes/servlet/*
is used by some servlets to create a virtual temporary location to pass
in dynamic information for theme files (*.css / *.xsl) which need to be
collected at runtime.

Example:
Servlet portlet-type-xsl used
   /themes/servlet/portlet-type/* 
to pass in at runtime a list of actually installed portlet's default xsl
templates to use as fallback.