Monokai code style and coal dark mode

[joenash]

Pull Request Description

This pull request fixes #27 and #11.
For 11: code colour scheme changed to Monokai. Code scheme is now consistent across all 3 colour schemes.
- Most of the changes are done in rouge-highlight-colorful.css (colorful being the name of the old theme, future refactor should rename this)
- There are some changes (namely the background of the code block in the default theme) made in the minimised style.css.
- For the sepia and dark themes, these changes are made in website.css.
For 27: In process of converting to Coal, translating between the css classes in mdbook and those in gitbook, got a bit more work to do to get full coverage.

Checklist

Please include the following checklist in your PR:

• The documentation has been updated if necessary.
• All code has been tested where possible.

[joenash]

I think this is good to go for now