.htaccess mod_rewrite and navigating back directories

Question

I have what may or may not be a peculiar problem, I need to point the htaccess file back three directories before finding the directory I am looking for. My understanding of navigating directories using mod_rewrite is limited, so I wanted to ask how to write an htaccess file that will do this:

URL: example.com/css/styles.css
Accesses file in path: root/app/webroot/css/styles.css

to this:
URL: example.com/css/styles.css
Accesses file in path: root/app/webroot/css/.htacces redirects to
root/themes/themeName/css/styles.css

Any help is very much appreciated, thank you.