theme/
└─inc/
  ├─static.php     # wp_enqueue_style() and wp_enqueue_script()
  ├─menus.php      # register_nav_menus()
  ├─hooks.php      # add_filter() and add_action()
  ├─helpers.php    # Helper functions and classes
  ├─widgets/       # Theme widgets
  │ ├─{widget-name}/
  │ │ ├─class-widget-{widget-name}.php # class Widget_{Widget_Name} extends WP_Widget { ... }
  │ │ ├─some-file.php
  │ │ └─some-dir/
  │ │   └─...
  │ └─...
  └─includes/      # All .php files are auto included (no need to require_once)
    ├─some-file.php
    └─...