_s: Update wpcom compat file to wrap contents in a hooked callback.

inc/wpcom.php

@@ -2,16 +2,22 @@
 /**
  * WordPress.com-specific functions and definitions.
  *
+ * This file is centrally included from `wp-content/mu-plugins/wpcom-theme-compat.php`.
+ *
  * @package _s
  */
 
-global $themecolors;
-
 /**
- * Set a default theme color array for WP.com.
+ * Adds support for wp.com-specific theme functions.
  *
  * @global array $themecolors
+ * @return void
  */
+function _s_wpcom_setup() {
+	global $themecolors;
+
+	// Set theme colors for third party services.
+	if ( ! isset( $themecolors ) ) {
 		$themecolors = array(
 			'bg'     => '',
 			'border' => '',
@@ -19,3 +25,6 @@ $themecolors = array(
 			'link'   => '',
 			'url'    => '',
 		);
+	}
+}
+add_action( 'after_setup_theme', '_s_wpcom_setup' );