Description

Custom CSS is being reverted when pressing save.

Step-by-step reproduction instructions

1. Make DISALLOW_UNFILTERED_HTML defined and truthy, e.g. by adding config: { "DISALLOW_UNFILTERED_HTML": true }, to the top level in .wp-env.json and starting with wp-env
2. Open the site editor
3. Open the global styles sidebar
4. Click the new 'Custom CSS' feature at the bottom of the page
5. Paste some style rule in like h1.wp-block-heading { text-decoration:underline; color: orange; }
6. Notice that the editor to the left now makes headings orange and underlined ✅
7. Press Save and then Save again once the pre-save prompt has loaded

Screenshots, screen recording, code snippet

The save HTTP request included the css values:

{
	"id": 6,
	"styles": {
		"blocks": {<CUT_FOR_BREVITY>},
		"color": {
			"gradient": "var(--wp--preset--gradient--base-secondary-base) no-repeat"
		},
		"css": "h1.wp-block-heading { text-decoration:underline; color: orange; }",
		
		"typography": {
			"fontFamily": "var(--wp--preset--font-family--dm-sans)"
		}
	}
}

The response however did not include a "css" property.

Environment info

Using the wp-env from Gutenberg 14.8.01-rc.1, can also reproduce on current trunk (1b5994a)

With DISALLOW_UNFILTERED_HTML, or presumably admin on multisite install or using any role with edit_theme_options but w/o unfiltered_html

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes