bad syntax highlighting with arrow highlighting style, javascript disabled, and dark mode

[gordonwoodhull]

Our fix for flashing in dark mode in #12173 requires us to load both stylesheets and disable one in JavaScript at the top of the body.

Because of, we are susceptible to stylesheet bleed when JavaScript is disabled. The stylesheet for the default mode is always loaded second, but if it doesn't redefine everything, especially background colors, then styles from the non-default stylesheet can be seen.

One symptom is bad syntax highlighting with the default highlighting style in this document.

---
format: html
theme:
  light: united
  dark: slate
_quarto:
  tests:
    html:
      ensureHtmlElements:
        - []
        - ["link[href*=\"-dark-\"]"]
---

## Hello

```markdown
![asdf](asd.png)
```

When JavaScript is disabled, this looks like

This only seems to be an issue with arrow, but arrow is the default.

A workaround is to use a different highlighting-style. I tested the styles listed as working with light and dark mode, and the others all work.

(Note: Highlighting for links has an intentionally dark background in atom-one, gruvbox.)

[cscheid]

This only seems to be an issue with arrow, but arrow is the default.

Proposal: let's patch arrow-light and arrow-dark.

[cderv]

For more context, the arrow theme is our own theme IIRC. It was designed based on several others as a good accessibility tool, first in R Markdown ecosystem, and then in Quarto when it started where a dark one has been introduced. So we can modify it at will.

[cscheid]

I think that the main thing that we should fix is to remove the background color and bold style from the alert entry:

https://github.com/quarto-dev/quarto-cli/blob/main/src/resources/pandoc/highlight-styles/arrow-dark.theme#L4-L5

In my local experiments, this is a small change with a big improvement. I still don't like the arrow dark theme very much (and we're switching to a11y on quarto-web for the dark mode PR), but I think this change puts it on the good side of "acceptable".