Add theme-checkout option to GitHub Pages workflow

.github/workflows/github_pages.yml

@@ -22,6 +22,11 @@ on:
         default: ""
         description: "The GitHub repo URL of a custom theme to use, for example: 'https://github.com/seanh/sidecar.git'"
         type: string
+      theme-checkout:
+        required: false
+        default: ""
+        description: "Git ref (branch, tag or commit) of the theme repo to checkout. This can be used to pin the version of your theme. If not specified defaults to the theme repo's default branch."
+        type: string
       python:
         required: false
         default: "3.14"
@@ -61,9 +66,12 @@ jobs:
         uses: actions/setup-python@v5
         with:
           python-version: ${{ inputs.python }}
-      - name: Checkout theme
+      - name: Clone theme
         if: ${{ inputs.theme }}
         run: git clone '${{ inputs.theme }}' .theme
+      - name: Checkout theme ref
+        if: ${{ inputs.theme && inputs.theme-checkout }}
+        run: git -C .theme checkout '${{ inputs.theme-checkout }}'
       - name: Configure GitHub Pages
         id: pages
         uses: actions/configure-pages@v5