Classic themes: Only load classic theme button styles for themes that don't use theme.json

[scruffian]

In #44334 we added fallback styles for buttons to classic themes. This approach won't work for classic themes that use theme.json - the fallback styles should not be for all classic themes, only for those that don't opt in to theme.json.

This changes the conditions when we load this classic theme fallback - now it only loads on themes that don't have theme.json support.

Testing instructions:

• Switch to Twenty Ten
• Add some button blocks
• Add a theme.json file that looks like this:

{
	"version": 2,
    "settings": {
        "layout": {
           "contentSize": "720px",
           "wideSize": "960px"
       }
   },
	"styles": {
		"elements": {
			"button": {
				"color": {
					"background": "yellow",
					"text": "white"
				},
				"border": {
					"color": "white",
					"radius": "70px",
					"style": "solid",
					"width": "4px"
				},
				"typography": {
					"fontWeight": "700"
				}
			}
		},
		"color": {
			"background": "yellow",
			"text": "white"
		}
	}
}

• Confirm that the button background is still black on trunk in the editor:

- Now switch to this branch and confirm that in the editor the buttons have a yellow background:

cc @WordPress/block-themers

[jffng]

The testing instructions check out, but I'm a bit confused about how the original PR (#44334) is intended to work.

For example, in TT (no theme.json added):

No Gutenberg	w/ Gutenberg trunk and this PR

Why have the button styles changed?

[scruffian]

Great catch @jffng. I added a commit to address part of that, but I think we also need to fix it in the theme, which I did in the wordpress-develop PR here: WordPress/wordpress-develop#3481

Curious what you think of that approach.

[MaggieCabrera]

We might need to patch more than TTwenty. This is TwentyThirteen:

[scruffian]

Twenty Thirteen looks ok to me:

[MaggieCabrera]

Twenty Thirteen looks ok to me

I meant with the theme.json but I guess that's normal, since the theme CSS will override it. It's still working in the background color so it is applying as much as the theme allows

[MaggieCabrera]

I tried a bunch of classic themes and the wordpress-develop PR seems to work as expected

[scruffian]

I'm going to hold off merging this until we see what the outcome of WordPress/wordpress-develop#3481 is

[t-hamano]

This PR seems to target WordPress 6.1, but the latest Gutenberg now supports WordPress 6.2 and above. Also, it looks like the backport PR to the core is already committed.

Is it possible to close this PR and #48750?

[scruffian]

This has been patched in core so I think we can close it.