Source

File: wp-includes/class-wp-customize-manager.php

protected function update_stashed_theme_mod_settings( $inactive_theme_mod_settings ) {
		$stashed_theme_mod_settings = get_option( 'customize_stashed_theme_mods' );
		if ( empty( $stashed_theme_mod_settings ) ) {
			$stashed_theme_mod_settings = array();
		}

		// Delete any stashed theme mods for the active theme since they would have been loaded and saved upon activation.
		unset( $stashed_theme_mod_settings[ $this->get_stylesheet() ] );

		// Merge inactive theme mods with the stashed theme mod settings.
		foreach ( $inactive_theme_mod_settings as $stylesheet => $theme_mod_settings ) {
			if ( ! isset( $stashed_theme_mod_settings[ $stylesheet ] ) ) {
				$stashed_theme_mod_settings[ $stylesheet ] = array();
			}

			$stashed_theme_mod_settings[ $stylesheet ] = array_merge(
				$stashed_theme_mod_settings[ $stylesheet ],
				$theme_mod_settings
			);
		}

		$autoload = false;
		$result   = update_option( 'customize_stashed_theme_mods', $stashed_theme_mod_settings, $autoload );
		if ( ! $result ) {
			return false;
		}
		return $stashed_theme_mod_settings;
	}