Variable scopes and colours

[ahosgood]

Added

• "Thick" and button border widths are customisable

Changed

• Improved markup of warning component
• --page-background is now --background

Deprecated

• Deprecated support for the tna-template--high-contrast-theme class - high contrast themes can be used by setting a preference for higher contrast in the operating system
• colour-font() no longer has the ability to define the default palette

Removed

• Fallbacks for custom CSS properties are defined in the var() function rather than in a separate property - this removes any colouring support for browsers without custom CSS properties

Fixed

• Fixed column margin removal classes
• Added missing IDs to some form elements so they can be linked top from the error summary