README <picture> theme selection follows OS appearance instead of GitHub theme

[kumarshreshtha]

🏷️ Discussion Type

Bug

💬 Feature/Topic Area

Other

Body

README images selected using prefers-color-scheme follow the operating
system's appearance rather than the theme selected on GitHub.com.

Steps to reproduce

1. Add this to a repository README, using three distinct images:

   <picture>
     <source media="(prefers-color-scheme: dark)" srcset="dark.svg">
     <source media="(prefers-color-scheme: light)" srcset="light.svg">
     <img src="fallback.svg" alt="Theme selection test">
   </picture>

2. Set the OS appearance to light and GitHub's appearance to dark.

3. Open the README: the light image is displayed.

4. Set the OS appearance to dark and GitHub's appearance to light.

5. Open the README: the dark image is displayed.

Expected behavior

The image should match GitHub's selected theme.

Actual behavior

The image matches the OS appearance, even when GitHub uses the opposite
theme. The selected image is a image, not the fallback .

Additional observations

• Reproduced in two browsers.
• Reproduced with both SVGs and GIFs.
• Older commits whose theme switching previously worked now exhibit
  the same behavior.
• This appears to be a regression, but I haven't established when it began.

Environment

• OS and version: MacOS Tahoe (26.6.2)
• Browsers: Tried on Firefox (Zen), Chromium (Arc) and Chrome

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[DYNOSuprovo]

This behavior is not a bug in GitHub's Markdown parser, but rather the fundamental web standard design of HTML5 <picture> tags and browser media queries.

Here is why it happens and the official GitHub mechanism to make images follow GitHub's theme toggle:

---

Why <picture media=\"(prefers-color-scheme: ...)\"> Follows the OS

1. The <picture> element and its media attribute are evaluated natively by the browser rendering engine, completely outside of GitHub's DOM and CSS classes.
2. The browser's prefers-color-scheme CSS media query strictly inspects the host operating system's theme (via window.matchMedia).
3. When you override GitHub's theme in your user settings (e.g. setting GitHub to "Dark" while macOS is set to "Light"), GitHub toggles data attributes on the root HTML tag (<html data-color-mode=\"dark\">). It does not (and cannot) change your operating system's system-level media query.
4. Consequently, the browser's native parser evaluates <source media=\"(prefers-color-scheme: light)\"> as true and renders the light asset.

---

The Official GitHub Solution: #gh-dark-mode-only and #gh-light-mode-only

GitHub officially created theme context URL fragments specifically to solve this problem. When you append #gh-dark-mode-only or #gh-light-mode-only to your image URLs, GitHub's stylesheet toggles visibility based on GitHub's active theme attribute rather than the OS media query.

Standard Markdown Syntax:

![Theme Logo](dark.svg#gh-dark-mode-only)
![Theme Logo](light.svg#gh-light-mode-only)

HTML Syntax (with Links / Centering):

<p align=\"center\">
  <img src=\"dark.svg#gh-dark-mode-only\" alt=\"Dark Theme Banner\" width=\"600\">
  <img src=\"light.svg#gh-light-mode-only\" alt=\"Light Theme Banner\" width=\"600\">
</p>

---

How GitHub Enforces This Under the Hood

GitHub injects global styles matching the URL hashes against the data-color-mode attribute:

[data-color-mode="dark"] [src$="#gh-light-mode-only"],
[data-color-mode="light"] [src$="#gh-dark-mode-only"] {
  display: none !important;
}

This ensures that:

• If a user sets GitHub to Dark on a Light OS: dark.svg is displayed.
• If a user sets GitHub to Light on a Dark OS: light.svg is displayed.
• If GitHub is set to Sync with system: it automatically tracks the OS appearance as before.

Switching from <picture> to the #gh-*-mode-only fragments will immediately give you the expected theme-synchronized behavior! If this explains the behavior and solves your README styling, please feel free to mark this as the accepted answer.

[kumarshreshtha]

Thanks a lot! It works perfectly.

[ibr7780a]

f I want to talking about something again

[ibr7780a]

f I want to talking about something again