Change to use inner margins instead of paddings, closes #148

This makes the rows go all the way to the edge, and adds margins to the text instead, so backgrounds/lines look better. Closes #148.
darken() allow you to use the same color value multiple places, so you can search for "#fee". _not_ using darken() will not allow you to reuse the same value. Actually, "#fee" should probably be stored in a variable.

That's the whole point of using sass: Allowing functions to do the heavy lifting.