You start tomcat from some directory - which is the $cwd for tomcat. You can specify any path relative to this $cwd.
suppose you have
home
- tomcat
|_bin
- cssStore
|_file.css
And suppose you start tomcat from ~/tomcat, using the command "bin/startup.sh".
~/tomcat becomes the home directory ($cwd) for tomcat
You can access "../cssStore/file.css" from class files in your servlet now
Hope that helps, - M.S.