Try has_tag for it.

function customstyles() { 
  if ( has_tag( 'circulair' ) ) {
    //Register and enqueue the stylesheet for tag-circulair.
    wp_register_style( 'tag-circulair', get_template_directory_uri() . '/layout-circulair/style.css' );  
    wp_enqueue_style( 'tag-circulair' );
  } else {
    //Register and enqueue the default stylesheet.    
    wp_register_style( 'styles', get_stylesheet_uri() );
    wp_enqueue_style( 'styles' );  
  }
}
add_action( 'wp_enqueue_scripts', 'customstyles' );

Hope this will helps you.

Try has_tag for it.

function customstyles() { 
  if ( has_tag( 'circulair' ) ) {
    //Register and enqueue the stylesheet for tag-circulair.
    wp_register_style( 'tag-circulair', get_template_directory_uri() . '/layout-circulair/style.css' );  
    wp_enqueue_style( 'tag-circulair' );
  } else {
    //Register and enqueue the default stylesheet.    
    wp_register_style( 'styles', get_stylesheet_directory_uri() );
    wp_enqueue_style( 'styles' );  
  }
}
add_action( 'wp_enqueue_scripts', 'customstyles' );

Hope this will helps you.