Remove themes from config

- A simple local variable suffices.
- No need to write this to .yo-rc.json.
Use yeoman-generator v0.18.0

* Update to use new fs API
* Had to change templates that generate templates as there is a
regression bug in yeoman-generator
(https://github.com/yeoman/generator/issues/700)
Fix test

The option was not set, since `useSass` was `true`.