fix: replace deprecated #gh-dark-mode-only fragments with <picture> element in Responsive Card Theme preview

[Gilmoursa]

What

Two locations in the README had broken images caused by the deprecated #gh-dark-mode-only / #gh-light-mode-only URL fragment syntax, which GitHub no longer renders:

1. The preview at the top of the Responsive Card Theme section (had an additional malformed double anchor in the link href, e.g. #responsive-card-theme#gh-dark-mode-only)
2. The live example inside the "Use GitHub's theme context tag" <details> block

Fix

Replaced both instances with <picture> elements using prefers-color-scheme media queries — the same approach the README itself documents as the correct modern method in the "Use GitHub's new media feature" subsection.

The catbox.moe screenshots in the Vercel setup guide were also audited and confirmed still live (all returning HTTP 200).

Testing

Verified locally that the <picture> syntax matches the working examples already present in the "Use GitHub's new media feature" subsection.

[vercel]

@Gilmoursa is attempting to deploy a commit to the github readme stats Team on Vercel.

A member of the Team first needs to authorize it.

[martin-mfg]

Thanks for the PR, but it seems the problem with the mentioned images is not the dark-mode/light-mode fragment. The problem is that the server is down and doesn't serve any images. We can see e.g. here that the dark-mode/light-mode fragments work.

Or did I miss something?

[Gilmoursa]

Good catch on the server outage — you're right that the preview images are currently broken because of that, not purely the fragment syntax. That said, GitHub's documentation does explicitly mark #gh-dark-mode-only / #gh-light-mode-only as deprecated in favor of the <picture> element, so the fix is still valid as a proactive cleanup even if the images won't render until the server is back up. Happy to close if the project prefers to hold off until the deployment situation is resolved.

[martin-mfg]

Good point. I agree that we should anyway recommend the <picture> approach.

I have created a PR in github-stats-extended. Because maintenance of this project has moved to https://github.com/stats-organization/github-stats-extended (see #3851 (comment) for context).

GitHub's documentation does explicitly mark #gh-dark-mode-only / #gh-light-mode-only as deprecated

Could you point me to this documentation please?

[Gilmoursa]

Sorry for the overstatement. It's not explicitly deprecated. The sources I was using were third-party blog posts, not GitHub's docs. The approach is what GitHub recommends as the modern method, but it's not deprecated.https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/quickstart-for-writing-on-github#adding-an-image-to-suit-your-visitors