Correct mislabeled mixin names

Currently, the names of the `solarized-light` and `solarized-dark` mixins are incorrectly swapped: the light mixin specifies a dark background color and matching text color, while the dark mixin specifies a light background color and matching text color. I've corrected the mixin names by re-swapping them.

As an aside, thanks for publishing this – it's been useful, at least for me :)