Customizer: spinner animated GIF and high CPU usage

[afercia]

As a best practice and as already done for the Theme Installer and the ThickBox modal, see tickets 33322 and 33311, when the spinner animated GIF is used as a background image, the element the background is applied to should be ​promoted to a new layer and ​the re-painted area should be reduced to the smallest possible one.

#customize-container and #customize-preview should just reuse the same CSS used for the Theme Installer.

[afercia]

First pass. In the screenshots below, the re-painted area in Chrome before and after the patch.
@celloexpressions should #customize-container keep a white background or this would be an opportunity to change it to #f1f1f1 as the standard background color?

[afercia]

Note: same solution was used also for the authentication modal dialog in #34951.

[afercia]

When the Customizer is loaded through the "customize container", the quick change from a white background to a gray background doesn't look so nice to me, see below. I'd propose to change the white in the standard #f1f1f1 background color.

[afercia]

For consistency, I'd propose to make also the Theme Installer preview background #f1f1f1.

[afercia]

In 36518:

Customizer: reduce the spinner re-painted area to the smallest possible one.

Also, changes the "customize container" and the theme installer preview
background from white to the default body #f1f1f1 background.

Fixes #35649.

[celloexpressions]

Sorry about the late reply, but yes, the #f1f1f1 is definitely better.

[afercia]

Replying to celloexpressions:

Sorry about the late reply, but yes, the #f1f1f1 is definitely better.

Thanks @celloexpressions worth noting in the Theme Installer there's still a white background visible for Internet Explorer and Edge users but that's caused by a different issue, the "white flash while iframe loads", see ​https://css-tricks.com/prevent-white-flash-iframe/