Mini Cart Contents: background doesn't work on the frontend with Gutenberg disabled

[dinhtungdu]

Describe the bug

Using WordPress 5.9 beta, with the Gutenberg disabled, admins can still edit and save the background color of the Mini Cart Contents block in the site editor. But the color doesn't apply on the front-end.

To reproduce

Steps to reproduce the behavior:

1. Update to WordPress 5.9 beta.
2. Disable Gutenberg.
3. Go to Appearance > Site Editor > Template Parts > Mini Cart.
4. Select the Mini Cart Contents block, change the background.
5. See the background changed in the site editor.
6. Save the block, see the background doesn't work on the front end.

Expected behavior

The background should work with WordPress 5.9.

Additional context

The background color works with Gutenberg Site Editor: /wp-admin/themes.php?page=gutenberg-edit-site. So the issue is already fixed in upstream Gutenberg.

Original comment by @Aljullu:

Something I noticed while doing some more testing is that in WP 5.9 beta 3 with Gutenberg disabled, I can check the color in the editor, but the new color is not applied in the frontend. @dinhtungdu can you reproduce that too? If so, it would be great to create an issue about that so we can investigate it further.