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.
FYI: The format of a length value (denoted by <length> in this specification) is a <number> (with or without a decimal point) immediately followed by a unit identifier (e.g., px, em, etc.). After a zero length, the unit identifier is optional.