Customizer: body element needs a CSS class for the color scheme

[wildworks]

The dashboard injects CSS classes like admin-color-{slug} into the body element based on the Admin Color Scheme setting, but the Customizer does not.

Gutenberg has ​a mixin that allows us to apply a color scheme to components based on this CSS class.

As a result, the color scheme is not applied to the command palette in the Customizer screen.

Steps to Reproduce

• Go to Users > admin
• Select any scheme other than the default scheme
• Activate Twenty Twenty-One
• Go to Appearance > Customize
• Launch the Command Palette (Cmd+K on Mac or Ctrl+K on Windows)
• Type some characters
• The selected command item has a default (fresh) color

[wildworks]

Launch the Command Palette in the Customizer Screen

Trac ticket: https://core.trac.wordpress.org/ticket/64415

[soyebsalar01]

I tested after patch and it is working properly
See the below Image:
​https://ibb.co/BVDVDWhN

[wildworks]

In 61385:

Customizer: Add admin color scheme class to body element.

This changeset injects a CSS class called admin-color-{slug} into the body element in the Customizer screen based on the admin color scheme setting.

Props soyebsalar01, westonruter, wildworks.
Fixes #64415.