Feature suggestion: map (additional) Yaml variables to CSS :root

Description

I'm suggesting that it could be useful to map a few additional CSS variables from Yaml to the CSS :root variables. Currently, there are already some variables defined such as:

:root {
--quarto-body-bg: #ffffff;
--quarto-body-color: #212529;
--quarto-text-muted: #6c757d;
--quarto-border-color: #dee2e6;
--quarto-border-width: 1px;
--quarto-border-radius: 0.25rem;

--quarto-font-monospace: SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
}

and, for example, the document's fontcolor, monofont, linkcolor, etc, can be set in the Yaml. I believe it would be useful to add a couple of such mappings, for example to define a "accent" / "highlighting" colour, as a minimal theming system. It's common for a document to have one main colour (black font), and another colour for titles, buttons, etc. This kind of logic is used in other systems, such as Beamer themes or ggplot2 themes. Colour palettes can even be derived from this parent element by CSS transformations, e.g.

color: color-mix(in srgb, var(--quarto-highlight) 20%, transparent);

to create a semi-transparent, lighter shade.

I'm not sure how many of these mappings might make sense – presumably just very few, otherwise it'll get too complicated to follow the possible interactions between Bootstrap and (S)CSS variables.