Use named colours for colour variables. Also purges close matching colours.

As per suggestion from /frontend and blog team.
Remove missing %body-text references

Was posting a warning in Rails server log.

```The selector "%body-text" was not found.
This will be an error in future releases of Sass.
Use "@extend %body-text !optional" if the extend should be able to
fail.
```
Add whitespace between colour variable lines
Merge pull request #64 from moneyadviceservice/use-named-colours

Use named colours and purge close-matching colours