style.css: Use 1em as base size for text

WHAT IS THIS?  A SCHOOL FOR ANTS?!
style.css: Update formatting of code blocks

initially dug that yellow background cuz I though it looked snazzy
on the Arch Linux website, but meh; we can set them apart based on
monospace alone.
style.css: Slide some more black colors up the scale

Gives more contrast to stuff that matters, like headers and body
text.