Avoid repeated overwrites of config['extra_*']

Fixes #133
Oh, I think I see the issue: while `None` is indeed the default value in config.py (near the top), in `validate_config`, it turns a value of `None` into a list of all the CSS files in the docs directory. Presumably, if this results in an empty list, your `if` expression evaluates to `False`, and then the empty list gets turned into a `None` value.