bettering support for dark ez-themes

This is an incremental improvement to the support of themes with dark backgrounds using the ez-themer in the admin menu. This improves upon some basic SASS logic for whether or not a color should be lightened or darkened. Still big improvements to be made, but this is a good start.

Also pulled variables out of box-shadow colors and set them to semi-opaque black, because having shadow colors change with a theme has some really poor results.