Changes to skip link CSS

Included:
* Decoupling the skip link CSS from the markup.
* Reducing the accessibility-specific CSS to hiding the skip links by default and showing them on focus.
* Moving presentation-specific (visuals and layout) CSS properties from the accessibility additions to the theme CSS.
* Updating the layout templates to use the new, decoupled skip link CSS.

Excluded:
* Cleaning up the presentation-specific (visuals and layout) CSS properties for the skip links.