Bring the install/update screens in line with the new admin design. This affects:

* Creating a wp-config.php file
* Installation
* Database upgrade
* Database repair
* Readme.html
* Network sites actions

What's changed:

* Bring background/text/link/border colours in line with mp6
* Bring fonts in line with mp6 (see note below)
* Switch to device-width for the viewport and tweak margins etc so the layout works nicely on all device sizes
* Switch to the new vector WordPress logo in the header to match the log in screen
* Force table cells into rows in the <782px media query

Fixes #25951, props johnbillion.


Built from https://develop.svn.wordpress.org/trunk@26298


git-svn-id: http://core.svn.wordpress.org/trunk@26203 1a063a9b-81f0-0310-95a4-ce76da25c4cd