Use classes instead of IDs and use the new <main> element.

The IDs are kept to keep compability with child themes.

* #content -> .site-content
* #main -> main.site-main
* #footer -> .site-footer
* #branding -> .site-header
LESS: Variables

* A lot of new variables
* New names: @grayLight -> @gray-light
* @black and @white remove
* Font size variables
* Component variables
LESS: Text selection mixin
LESS: Combine the two button mixins into one
LESS: New label mixin
LESS: Update the mixins to use the new variables
LESS: Update the files to use the new variables, mixins and classes
Add the compiled CSS files