mixin extends elements that do not work inside the @media

[Anderson-Juhasc]

I think we could momentarily change the mixins that extends. To make use within media until the sass fix it.

I had the problem with the arrows which makes extensive use of @extend.

What do you think?

abs

[SamHasler]

It would also be helpful if everything was available as a mixin for use inside @media. I've had to make nav a mixin so that I could change how it looks at different breakpoints.

[csswizardry]

Please see #291.

Thank you.