Fix ability to preview Additional CSS changes in the Customizer

[westonruter]

Adding the Additional CSS from the Customizer to the global styles makes sense, except not in the Customizer preview. This is because wp-includes/js/customize-preview.js is supposed to dynamically update the STYLE#wp-custom-css element with the new stylesheet whenever it changes. So this PR prevents putting the Additional CSS into the global styles when in the Customizer preview context.

This issue was introduced in #58991

Screencast

Before

Screen.Recording.2025-06-14.at.13.28.10.mov

After

Screen.Recording.2025-06-14.at.13.31.21.mov

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[westonruter]

As noted in WordPress/wordpress-develop#9000 (comment):

Actually, I think this change can be undone. It's a deprecated function anyway so it is not used. And the logic here is not right as it should rather be doing something like this:

$custom_css = '';
if ( ! is_customize_preview() ) {
	// Don't enqueue Customizer's custom CSS separately.
	remove_action( 'wp_head', 'wp_custom_css_cb', 101 );

	$custom_css .= wp_get_custom_css();
}

$custom_css .= wp_get_global_styles_custom_css();

But again, given that this function is deprecated it doesn't seem necessary to make this change.

Suggested change

	if ( ! wp_is_block_theme() || is_customize_preview() ) {
	if ( ! wp_is_block_theme() ) {