improve css customizability

This pull request just makes it a little more easy/obvious how and what to override when deploying a new instance. 

This also establishes a best practice of keeping deployment-specific CSS overrides in a file (local.css) which should never have merge conflicts when updating from CfA's Citygram sometime in the future.

It also provides an example of what a typical deployment would want to override (the Seattle header image) and documents the expected size.