Support dark/light theme images in markdown (#36922)

This PR matches GitHub's behavior more closely on how to render Markdown
images in light/dark mode.
Images with source suffix `#gh-dark-mode-only` / `#gh-light-mode-only`
will only show when the correct theme is requested.
Closes: #35545

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/webcomponents/index.ts

@@ -2,3 +2,14 @@ import './polyfills.ts';
 import './relative-time.ts';
 import './origin-url.ts';
 import './overflow-menu.ts';
+import {isDarkTheme} from '../utils.ts';
+
+function initPageThemeDarkLight() {
+  // Set page's theme color preference as early as possible, to avoid flicker of wrong theme color during page load.
+  const sync = () => document.documentElement.setAttribute('data-gitea-theme-dark', String(isDarkTheme()));
+  sync();
+  // Track system theme changes in case Gitea is using "auto" theme.
+  window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', sync);
+}
+
+initPageThemeDarkLight();