Toolbar: apply default color scheme to front-end admin bar.

Reapply front-end admin bar color scheme support that was reverted in
9027fd9 because the previous implementation issued an HTTP request via
wp_remote_get() on every front-end page load. Read the modern color scheme
stylesheet directly from the local filesystem instead, and extract only
rule blocks whose selector contains `#wpadminbar` so unrelated admin-only
styles (pointers, media library, theme browser, customizer, etc.) are
not leaked to the front end. Per-user color schemes are intentionally not
honored to keep the cost predictable.

See #64762.

Author: t-hamano

src/wp-includes/admin-bar.php

@@ -1462,3 +1462,27 @@ function _get_admin_bar_pref( $context = 'front', $user = 0 ) {
 
 	return 'true' === $pref;
 }
+
+/**
+ * Applies the default "modern" admin color scheme to the front-end admin bar.
+ *
+ * Per-user color schemes are not honored on the front end to avoid the
+ * per-request cost of resolving them.
+ *
+ * @since 7.0.0
+ */
+function wp_admin_bar_add_color_scheme_to_front_end() {
+	if ( is_admin() ) {
+		return;
+	}
+
+	$suffix = SCRIPT_DEBUG ? '' : '.min';
+	$css    = @file_get_contents( ABSPATH . "wp-admin/css/colors/modern/colors$suffix.css" );
+
+	// Extract only rule blocks whose selector contains `#wpadminbar`.
+	if ( ! $css || ! preg_match_all( '/[^{}]*#wpadminbar[^{}]*\{[^{}]+\}/s', $css, $matches ) ) {
+		return;
+	}
+
+	wp_add_inline_style( 'admin-bar', implode( '', $matches[0] ) );
+}

src/wp-includes/default-filters.php

@@ -700,6 +700,7 @@
 add_action( 'wp_body_open', 'wp_admin_bar_render', 0 );
 add_action( 'wp_footer', 'wp_admin_bar_render', 1000 ); // Back-compat for themes not using `wp_body_open`.
 add_action( 'in_admin_header', 'wp_admin_bar_render', 0 );
+add_action( 'admin_bar_init', 'wp_admin_bar_add_color_scheme_to_front_end', 0 );
 
 // Former admin filters that can also be hooked on the front end.
 add_action( 'media_buttons', 'media_buttons' );