tweaking the Sass per scss-lint rules

Per https://github.com/causes/scss-lint (but alas, it is Ruby and i cant find a libsass version that is Node compatible), so this is an optional local, dev thing if you want to use it.

Also tried my hand at converting hardcoded color values into variables.

Comments inline below.

r= @nchapman
Maybe we should just make this `button-hover-color` and do the calculation here. Thoughts?