Refactor code to match current conventions

This is an update to Vellum such that the CSS is written in a way that follows our current standards and conventions.

Status: **Opened for visibility**
Reviewers: @kpeatt @ry5n 
JIRA: **CSOPS-846**

## Changes
* Set headings to standard capitilization
* Max of three `=` or `-` when underlining headings
* Fixed property orders to match our linting rules
* Repositioned line comments
* Corrected line comments to be written using the numbered notation
* Cleaned up unnecessary third level headings
* Removed any placeholder classes
* Added a few extra headings for clarity
* Tried to make the variable's introductory bullet list a bit more readable
* Disable `*::before/after` by default due to Android 4.1.x performance issues, fixed #53
* Add `[hidden]` attribute styles (moved over from Stencil)