Add automated JSON schema validation

.github/workflows/validate-theme.yml

@@ -0,0 +1,37 @@
+name: Validate Themes
+
+on:
+    pull_request:
+        paths:
+            - '**/theme.json'
+            - '**/styles/*.json'
+            - '**/assets/fonts/*.json'
+
+env:
+    PR_PATHS: >-
+        '**/theme.json'
+        '**/styles/*.json'
+        '**/assets/fonts/*.json'
+    HUSKY: 0
+
+jobs:
+    validate-theme:
+        runs-on: ubuntu-latest
+
+        steps:
+            - name: Checkout
+              uses: actions/checkout@v4
+              with:
+                  fetch-depth: 2
+
+            - name: Setup Node
+              uses: actions/setup-node@v4
+              with:
+                  node-version-file: '.nvmrc'
+                  cache: 'npm'
+
+            - name: Install dependencies
+              run: npm ci
+
+            - name: Validate Themes
+              run: node theme-utils.mjs validate-theme $(git diff --name-only HEAD^ -- ${{ env.PR_PATHS }} | awk -F/ '{print $1}' | uniq | paste -s -d, -)