feat(github-action): add update-themes action

.github/workflows/update-themes.yml

@@ -0,0 +1,27 @@
+name: "Update Themes"
+
+on:
+  pull_request_target:
+    types: [ closed ]
+
+jobs:
+  main:
+    if: github.event.pull_request.merged == true
+    name: Update changelogs and versions
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          # pulls all commits (needed for lerna to correctly version)
+          fetch-depth: "0"
+          all_commits: true
+          all_tags: true
+
+      - uses: actions/setup-node@v2
+        with:
+          node-version: 12
+      
+      - run: git config --global user.email 41898282+github-actions[bot]@users.noreply.github.com
+      - run: git config --global user.name "github-actions[bot]"
+      - run: npm ci
+      - run: npm run update-themes