Customize admin Add new post page

This page structure is very complex. if you want to change editor style and function then go to "wp-admin\edit-form-advanced.php" line no 271 "the_editor($post->post_content);" function which point "wp-includes/general-template.php" line 1777. if you want to edit media button then look on this function "do_action( 'media_buttons' );" line 1808 in 'general-template.php'