[Proposal] Add `!default" to allow sass color overwrites

[jonashoechst]

Adding a !default tag to the defined variables allows the users to overwrite the variables in a file importing the theme.

This is especially useful when using the theme as a GitHub pages remote theme.

The change was also discussed and implemented for the cayman theme: pages-themes/cayman#11

Thank you for considering!

[daviddarnes]

I didn't know this was caused them to work like that! Will definitely add this in when I get a chance, thanks @jonashoechst

[jonashoechst]

Thanks for the enhancement. However you seem to have missed $accentColour, is this by accident?

[daviddarnes]

@jonashoechst urgh sorry, this was by accident. Hopefully get to it soon, however feel free to make a pull request to fix it! 😃