Use non-deprecated SCSS color function.

[ccorn]

This PR is to remove the following warning by rake doc:

DEPRECATION WARNING on line 166 of /_design.scss:
The operation `#1666A3 div 2` is deprecated and will be an error in future versions.
Consider using Sass's color functions instead.
http://sass-lang.com/documentation/Sass/Script/Functions.html#other_color_functions

My first attempt was to use darken($link-color, 50%) and that was wrong because apparently equivalent to adjust-color($link-color, $lightness: -50%), resulting in black. The original intent is equivalent to scale-color($link-color, $lightness: -50%) so I have used that.

Verified that the resulting CSS is the same as in the master branch.

[gettalong]

Thanks!