Fix theme and upgrade ml-trainer-microbit theme package (#626)

* Upgrade ml-trainer-microbit package

* Add a placeholder homepage-short-clip.mp4 for theme

Author: microbit-grace

.github/workflows/build.yml

@@ -39,7 +39,7 @@ jobs:
       - run: npm ci
         env:
           NODE_AUTH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-      - run: npm install --no-save @microbit-foundation/[email protected].75 @microbit-foundation/[email protected] @microbit-foundation/[email protected]
+      - run: npm install --no-save @microbit-foundation/[email protected].78 @microbit-foundation/[email protected] @microbit-foundation/[email protected]
         if: github.repository_owner == 'microbit-foundation'
         env:
           NODE_AUTH_TOKEN: ${{ secrets.GITHUB_TOKEN }}