Playground Previews: use correct github proxy URL

Changing where we make the call to download the theme, to make sure that
we're using stable code.

Additionally, add child theme handling.

Discussion around this:
- Automattic/themes#7719
- stoph/github-proxy#3

.github/scripts/create-preview-links.js

@@ -19,7 +19,7 @@ function createBlueprint(themeSlug, branch) {
 				step: 'installTheme',
 				themeZipFile: {
 					resource: 'url',
-					url: `https://github-proxy.com/partial/Wordpress/community-themes/${themeSlug}?branch=${branch}`,
+					url: `https://github-proxy.com/proxy.php?action=partial&repo=Wordpress/community-themes&directory=${themeSlug}&branch=${branch}`,
 				},
 			},
 			{
@@ -44,6 +44,27 @@ function getThemeName(themeSlug) {
 	return themeName;
 }
 
+/*
+ * This function reads the `style.css` file of a theme and returns the name of the parent theme.
+ * If the theme is not a child theme, it returns an empty string.
+ *
+ * @param {string} themeSlug - The slug of the theme to get the parent theme name of.
+ * @returns {string} - The name of the parent theme as defined in the `style.css` file.
+ */
+function getParentThemeName(themeSlug) {
+	const styleCss = fs.readFileSync(`${themeSlug}/style.css`, 'utf8');
+	const parentTheme = styleCss.match(/Template:(.*)/i);
+	const isChildTheme = parentTheme && '' !== parentTheme[1].trim();
+
+	if (!isChildTheme) {
+		return '';
+	}
+
+	return parentTheme && '' !== parentTheme[1].trim()
+		? parentTheme[1].trim()
+		: '';
+}
+
 /*
  * This function creates a comment on a PR with preview links for the changed themes.
  * It is used by `preview-theme` workflow.
@@ -56,25 +77,40 @@ async function createPreviewLinksComment(github, context, changedThemeSlugs) {
 	const changedThemes = changedThemeSlugs.split(' ');
 	const previewLinks = changedThemes
 		.map((themeSlug) => {
+			const parentThemeName = getParentThemeName(themeSlug);
+			const note = parentThemeName
+				? ` (child theme of **${parentThemeName}**)`
+				: '';
+
 			return `- [Preview changes for **${getThemeName(
 				themeSlug
 			)}**](https://playground.wordpress.net/#${createBlueprint(
 				themeSlug,
 				context.payload.pull_request.head.ref
-			)})`;
+			)})${note}`;
 		})
 		.join('\n');
+
+	const includesChildThemes = changedThemes.some(
+		(themeSlug) => '' !== getParentThemeName(themeSlug)
+	);
+
 	const comment = `
 I've detected changes to the following themes in this PR: ${changedThemes
 		.map((themeSlug) => getThemeName(themeSlug))
 		.join(', ')}.
 
 You can preview these changes by following the links below:
+
 ${previewLinks}
 
 I will update this comment with the latest preview links as you push more changes to this PR.
-
 **⚠️ Note:** The preview sites are created using [WordPress Playground](https://wordpress.org/playground/). You can add content, edit settings, and test the themes as you would on a real site, but please note that changes are not saved between sessions.
+${
+	includesChildThemes
+		? '\n**⚠️ Note:** Child themes are dependent on their parent themes. You will have to install the parent theme as well for the preview to work correctly.'
+		: ''
+}
 `;
 
 	const repoData = {