How to make theme elements customizable in wordpress? -

in wordpress there's menu option called 'appeareances'. want add (e.g.) submenu 'header' there , able customize header within wordpress (administration page).

from understood have have 'functions.php' file in theme folder. in 'functions.php' file write this:

<?php add_submenu_page( $parent_slug, $page_title, $menu_title, $capability, $menu_slug, $function); ?> 

of course adapted arguments (which i'm not entirely sure should since feel quite lost in this).

codex best friend ..

and codex say, if want add entry 'appearance' menu , use

for appearance: add_submenu_page( 'themes.php', ... );

or adapted case :

add_submenu_page( 'themes.php', 'header', 'header', 'edit_theme_options', 'customize_header','my_callback'); 

but ..:

also see > add_theme_page() 

like :

 add_theme_page( $page_title, $menu_title, $capability, $menu_slug, $function ); 

quoting codex :

parameters

$page_title (string) (required) text displayed in title tags of page when menu selected

    default: none  

$menu_title (string) (required) text used menu

    default: none  

$capability (string) (required) capability required menu displayed user.

    default: none  

$menu_slug (string) (required) slug name refer menu (should unique menu).

    default: none  

$function (callback) (optional) function called output content page.

tl;dr version

all above said in general custom menu entries, in specific case of custom header, need use :

add_theme_support( 'custom-header' ); 

see details here : http://codex.wordpress.org/custom_headers