Custom color scheme creates warning in the Site Editor

**Steps to reproduce**

1. Enable a block theme on any Altis version install.
2. Go to any page in the Site Editor (`wp-admin/site-editor.php`).
3. Open the JavaScript console.
4. See this warning: `colors-css was added to the iframe incorrectly. Please use block.json or enqueue_block_assets to add styles to the iframe.`

**Describe what you expected to see**

No warning.