Use `em` unit for `font-size` property
Remove unnecessary file
Rename variables

The variables for "Header", "Main", "Subcontent", and "Footer" are now
prefixed with its component name.
Organize partials by directory

  - [class]  placeholder classes
  - [footer] footer components
  - [header] header components
  - [main]   main components
  - [var]    variables
  - [widget] various widgets
    - [3rd-party] various widgets which depends on 3rd-party webservices
Decrease "Logo" on small screen

This changes also update "Site Navigation" layout.
Version 6.49.0