Update nightly-build.yaml

Author: erogluorhan

.github/workflows/nightly-build.yaml

@@ -8,7 +8,7 @@ on:
 jobs:
   build:
     if: ${{ github.repository_owner == 'ProjectPythia' }}
-    uses: ProjectPythia/cookbook-actions/.github/workflows/build-book.yaml@main
+    uses: ProjectPythia/cookbook-actions/.github/workflows/build-book.yaml@try_updated_theme
     with:
       environment_file: 'environment.yml'
       environment_name: pythia