Making improvements to new styles added in the MP6 merge, and addressing visual feedback from core team.

Improved and more consistent styling on the widgets & menus pages:

* Use the same visual treatment for both widgets and menus.
* Re-use existing styles from input:focus for draggable elements.
* Use the standard H3 size for widget area titles.
* Re-style the Widgets page drop zone to match drop zone from dashboard.
* Replace the gear icon on Widgets with the standard open/close arrows.
* Remove "Choose a sidebar" header from widget area chooser.

Make open/close arrows more consistent:

* Use the same arrow direction everywhere: pointing down when its container is closed, pointing up when open (following the Dashboard example).
* Set a consistent color for open/close arrows.
* Add a hover color for open/close arrows.
* Make open/close arrows permanently visible; :hover states don't exist on touch.

Make typography and color more consistent:

* Switch all declarations of #dedede to #ddd for simplicity's sake.
* Make H3s the same weight and color as H2s.
* Make select elements the same color as text inputs.
* Make paragraph text #444 so it doesn't look washed out on grey backgrounds.

General improvements:

* Eliminate the border between rows in settings tables.
* Make text and buttons line up in the Customizer.

See #25858.


Built from https://develop.svn.wordpress.org/trunk@26293


git-svn-id: http://core.svn.wordpress.org/trunk@26198 1a063a9b-81f0-0310-95a4-ce76da25c4cd