cause workflow to run on merged pr or manual trigger

.github/workflows/update.yml

@@ -2,9 +2,15 @@ name: Update theme
 
 on:
   workflow_dispatch:
+  pull_request:
+    types:
+      - closed
+    branches:
+      - main
 
 jobs:
   update_theme:
+    if: github.event.pull_request.merged == true || github.event_name == 'workflow_dispatch'
     runs-on: ubuntu-latest
     steps:
       - name: Check out repository