Background color should be applied to the body element instead of html

[michaelarestad]

Currently, the background colour for the admin is applied to html rather than body. This is not ideal. When applied to body, html inherits the colour. I know it sounds backwards, but that's how it works.

If a plugin attempts to add a background colour to body, there will be occasions when both the html background colour and the body background colour are visible at the same time.

I think simply using body is more flexible. I have included a patch.

[michaelarestad]

In 37321:

Component: Administration

Setting the background color on body is more flexible than on the html element. This patch assigns the background color to body instead of the html element.

Fixes #35314