Add submodule checkout for themes

.github/workflows/main.yml

@@ -24,6 +24,10 @@ jobs:
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
       - uses: actions/checkout@v2
+        
+      # Need to checkout submodules for themes
+      - name: Checkout submodules
+        uses: textbook/git-checkout-submodule-action@2.1.1
 
       # Runs a single command using the runners shell
       - name: Hugo Build