Update CUSTOM_THEME.md

Updated link to custom.css file which is now in the flavor's folder. The old link was to this [location] (https://github.com/openplans/shareabouts/tree/master/src/sa_web/static/css).
Merge pull request #107 from aplannersguide/patch-1

Fixed broken link to custom.css file in documentation.
Update dependency version for django-proxy

Version 1.0.2 fixes an issue with passing the upstream content-length through the proxy. Django should handle the content length itself, since it may decide to return the content with a different encoding than the upstream server.
Merge remote-tracking branch 'upstream/master'
Made email required