common: select style by class name from sass css

common: select style by class name from sass css

Combine all style options into light.css under s-* classes, enabled on html
root instead of loading a separate stylesheet.  Javascript for monochrome
media selection, otherwise identical behaviour.

Previously separate includes grow the resulting file from 16kB to 26kB.
Requires popular sassc for compilation and minification (replacing the
custom stripcss script).