### Bug description

Quarto currently doesn't support custom giscus themes, and instead restricts you to the prebuilt themes. If you input something other than those themes, e.g.:

```yaml
comments:
    giscus:
      theme: https://giscus.app/themes/custom_example.css
```

You get the following error:

```
Key theme has value https://giscus.app/themes/custom_example.css, which must be one of: `light`, `light_high_contrast`, `light_protanopia`, `dark`, `dark_high_contrast`, `dark_protanopia`, `dark_dimmed`, `transparent_dark`, `preferred_color_scheme`
```

giscus [supports custom themes](https://github.com/giscus/giscus/blob/main/ADVANCED-USAGE.md#data-theme) by supplying a URL to a CSS file as the value for the `data-theme` attribute, and it would be nice if Quarto supported this too.

### Checklist

- [ ] Please include a minimal, fully reproducible example in a single .qmd file? Please provide the whole file rather than the snippet you believe is causing the issue.
- [ ] Please [format your issue](https://quarto.org/bug-reports.html#formatting-make-githubs-markdown-work-for-us) so it is easier for us to read the bug report.
- [ ] Please document the RStudio IDE version you're running (if applicable), by providing the value displayed in the "About RStudio" main menu dialog?
- [ ] Please document the operating system you're running. If on Linux, please provide the specific distribution.
cscheid
2022-10-30T01:11:06Z
mccarthy-m-g
bug yaml-validation
v1.3
closed
Custom giscus themes not supported
https://github.com/quarto-dev/quarto-cli/issues/3105