Add classic theme styles as a static file not in a package

src/wp-includes/css/classic-themes.css

@@ -0,0 +1,18 @@
+/**
+ * These rules are needed for backwards compatibility.
+ * They should match the button element rules in the base theme.json file.
+ */
+.wp-block-button__link {
+	color: #ffffff;
+	background-color: #32373c;
+	border-radius: 9999px; /* 100% causes an oval, but any explicit but really high value retains the pill shape. */
+
+	/* This needs a low specificity so it won't override the rules from the button element if defined in theme.json. */
+	box-shadow: none;
+	text-decoration: none;
+
+	/* The extra 2px are added to size solids the same as the outline versions.*/
+	padding: calc(0.667em + 2px) calc(1.333em + 2px);
+
+	font-size: 1.125em;
+}

src/wp-includes/script-loader.php

@@ -3667,7 +3667,7 @@ function _wp_theme_json_webfonts_handler() {
 function wp_enqueue_classic_theme_styles() {
 	if ( ! WP_Theme_JSON_Resolver::theme_has_support() ) {
 		$suffix = wp_scripts_get_suffix();
-		wp_register_style( 'classic-theme-styles', "/wp-includes/css/dist/block-library/classic$suffix.css", array(), true );
+		wp_register_style( 'classic-theme-styles', '/' . WPINC . "/css/classic-themes$suffix.css", array(), true );
 		wp_enqueue_style( 'classic-theme-styles' );
 	}
 }
@@ -3687,7 +3687,7 @@ function wp_add_editor_classic_theme_styles( $editor_settings ) {
 		return $editor_settings;
 	}
 	$suffix = wp_scripts_get_suffix();
-	$classic_theme_styles = __DIR__ . "/css/dist/block-library/classic$suffix.css";
+	$classic_theme_styles = ABSPATH . WPINC . "/css/classic-themes$suffix.css";
 
 	// This follows the pattern of get_block_editor_theme_styles,
 	// but we can't use get_block_editor_theme_styles directly as it