[webrtc-priority] Spec is broken in dark mode (#18)

martinthomson has just created a new issue for https://github.com/w3c/webrtc-priority:

== 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.

Please view or discuss this issue at https://github.com/w3c/webrtc-priority/issues/18 using your GitHub account


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