Admin Bar CSS assumes body is not positioned

[joyously]

The Admin Bar is appended to the body of the page, and uses position: fixed for large windows. A space is made for it by using margin-top on the html tag.
This works fine until the media query for smaller windows changes it to position: absolute and there is user or theme CSS with body {position: relative}. The bar is then positioned relative to the body instead of html (leaving the margin visible and the bar over the top of body content).

I discovered this because I wanted to add a background overlay on the body, independent of the custom background, so I set the body position to relative and body:before to absolute.

A possible solution is to use body padding instead of html margin, but that might interfere more with theme CSS than the current method.

[SergeyBiryukov]

Related: #30055