Ensure that theme assets can be built

.github/workflows/ci.yml

@@ -57,6 +57,37 @@ jobs:
       - name: Verify the home page has the site name in it
         run: curl -s http://localhost:8000 | grep "Oliver Davies"
 
+  theme:
+    name: Build the theme assets
+
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        theme-path:
+          - web/themes/custom/opdavies
+
+    steps:
+      - name: Checkout the code
+        uses: actions/checkout@a81bbbf
+
+      - name: Setup node and npm
+        uses: actions/setup-node@56899e0
+        with:
+          node-version: 12
+
+      - name: Cache npm dependencies
+        uses: actions/cache@d974700
+        with:
+          path: node_modules
+          key: dependencies-npm-${{ hashFiles('package.json') }}
+
+      - name: Build the production theme assets
+        run: |
+          cd ${{ matrix.theme-path }}
+          npm clean-install
+          npm run production
+
   phpunit:
     runs-on: ubuntu-latest