Overriding the default bbpress theme

functions.php

/**
 * Set the theme compat theme properties
 *
 * @since bbPress (r3311)
 *
 * @global bbPress $bbp
 * @param BBP_Theme_Compat $theme
 * @uses current_theme_supports()
 */
function bbp_setup_new_theme_compat() {
	global $bbp;

	// Check if current theme supports bbPress
	if (!current_theme_supports( 'bbpress' ) ) {
			$theme          = new BBP_Theme_Compat();			
			$theme->name    = 'bbPress (Twenty Ten)'; /* Change to the name you need */
			$theme->version = '20110912'; /* Update these as you update the theme */
			$theme->dir     = WP_PLUGIN_DIR . '/twentyeleven-bbpress-child'; /* You might need to correct this */
			$theme->url     = plugins_url('twentyeleven-bbpress-child'); /* Ditto */

		// Set the theme compat globals for help with loading template parts
		$bbp->theme_compat->theme = $theme;
	}
}

add_action( 'bbp_init', 'bbp_setup_new_theme_compat',     9   );