Widget Customizer: Fix shift+click issue in Firefox to open a widget.

props westonruter.
see #27358.
Built from https://develop.svn.wordpress.org/trunk@27701


git-svn-id: http://core.svn.wordpress.org/trunk@27540 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Widget Customizer: Improve widget highlighting.

* Prefix box-shadow in inject_preview_css()
* Remove border-radius in inject_preview_css()
* Prevent hidden box-shadows in Customizer

see #27358.
Built from https://develop.svn.wordpress.org/trunk@27702


git-svn-id: http://core.svn.wordpress.org/trunk@27541 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Custom backgrounds: Do not print the default background color when it has been saved to the DB.

props obenland.
fixes #22030.

Built from https://develop.svn.wordpress.org/trunk@27703


git-svn-id: http://core.svn.wordpress.org/trunk@27542 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Custom backgrounds: Always cause a preview in the customizer.

Omitted from [27703].

props obenland.
see #22030.

Built from https://develop.svn.wordpress.org/trunk@27704


git-svn-id: http://core.svn.wordpress.org/trunk@27543 1a063a9b-81f0-0310-95a4-ce76da25c4cd