Cleanup style modules

Extract mixins
  - 'clearfix'
  - 'content block'
  - 'opaque-on-focus'

Style components use local variables
  - groups values repeated throughout a component
  - locals are created at the top from globals
  - style declarations use locals
  - some exceptions ($size, etc.) where not appropriate

Treat major page sections consistently
  - clearfix on banner, main, contentinfo

Specify font-weights explicitly for 'body' and 'bold' text
  - use numerical values
  - enables greater control

Reduce visual impact of footer content
  - lighten text colour

Specify local font stack based on Avenir Next