Update Cheatsheet print styles (#1665)

Main changes:

- all background color removed in order to avoid potential photocopying issues;
- more color from the main styles' text preserved in order to keep more character;
- H3 horizontal rules preserved, at 2px instead of 1px to differentiate from 1px tables and code boxes;
- paragraphs' and lists' borders removed, and tables' left/right borders removed, in order to lessen the "boxiness" (this allows for the H3 rules to make more visual sense).

The "screen" media query is removed from the main cheatsheet styles, allowing them to apply to the printed version in order to reduce repetition of setting font, color, etc.

Increases use of .content-inner selector namespace to be safe.

Author: DavidOliver