Generated html from deno doc should allow some customization

[RickCogley]

The current deno doc command in 2.4.3 generates html with a nav bar that appears to be transparent. This is not the greatest look I think, since as you scroll, the api docs site's name overlaps the text and it's just hard to read or see, as you can see in the pasted screenshot.

Is there a reliable way to control this? I think if we could specify a css to use, or if there were some options to control the css being used in the generation (deno doc --html --css:/path/to/custom.css etc, or a way to specify this in deno.json), that it would give a better and more professional result.