Twenty Fourteen: let authors customize the green "accent" color by changing to a different hex value in the Customizer. Props celloexpressions.

Also add an ID attribute to all `style` elements.

See #25220.
Built from https://develop.svn.wordpress.org/trunk@25418


git-svn-id: http://core.svn.wordpress.org/trunk@25343 1a063a9b-81f0-0310-95a4-ce76da25c4cd