Spec is broken in dark mode

The background rule for the editor's draft is:

```css
background: url("https://www.w3.org/StyleSheets/TR/logo-ED") top left no-repeat white; 
```
It should be:

```css
background: url("https://www.w3.org/StyleSheets/TR/logo-ED") top left no-repeat var(--bg);
```

@dontcallmedom probably knows where this belongs, which isn't this repo, I suspect.