Structuring Nav Menu and Background Custom page (Help)

i´m creating theme is my first theme and i have a problem with panel nav menu of wordpress, I´m creating the order of nav menu in the panel wordpress and does not order, i was make page and automatically the page add to nav menu, i need delete a page of menu and doesnt not eliminate.

this is my header.php

 !DOCTYPE html
html lang="?php language_attributes () ?"
head
    meta charset = "?php bloginfo ('charset');?"
    meta name="viewport" content="width=device-width, initial-scale=1.0"
    title?php wp_title (); ?/title
    ?php wp_head(); ?
/head
body class="?php body_class(); ?"
    div id="app-yahcrea" class="app-yahcrea"
        !-- Comienza el menu --
        header class="header-yahcrea"

            div class="content-header-yahcrea"
                div id="menu-icon" class="menu-icon"
                    span class="icon-menu"/span
                    span class="icon-menu"/span
                    span class="icon-menu"/span
                /div
                div class="menu-mobile" id="menu-mobile"

                    nav class="mobile-nav-yahcrea"

                        ?php 
                        wp_nav_menu(array(
                            'container' = true,
                            'items_wrap' = 'ul id="menu-top"%3$/ul',
                            'theme_location' = 'Primary',
                            'walker' = new Walker_Nav_Menu

                        ));
                    ?
                    /nav

                /div

                div class="logo-yahcrea"  ?php 
                $custom_logo_id = get_theme_mod( 'custom_logo' );
                    $logo = wp_get_attachment_image_src( $custom_logo_id , 'full' );
                    if ( has_custom_logo() ) {
                        the_custom_logo();
                    } else {
                            echo 'a href="'.home_url().'"img width="64" height="64"src="'. get_stylesheet_directory_uri() . './assets/images/PerfilFblanco.png' .'"/a';
                    }
               ?/div
                nav class="nav-yahcrea"    

                    ?php 
                    if(has_nav_menu('primary')){
                        wp_nav_menu(
                            array(
                                'theme_location' = 'Primary',
                                'container' = 'div',
                                'container_class' = 'collapse navbar-collapse',
                                'container_id' = 'navbarSupportedContent',
                                'items_wrap' = 'ul class="navbar-nav ml-auto text-center"%3$s/ul',
                                'menu_class' = 'nav-item',
                                'echo' = true,
                                'fallback_cb' = 'wp_page_menu', // en caso de que el menu no exista cargar wp_page_menu
                                'before' = '', // texto antes del texto del enlace.
                                'after' = '', // texto despues del texto del enlace.
                                'link_before' = 'span', // a href=""span ....
                                'link_after' = '/span', // /span/a
                                'item_spacing' = 'preserve', // preserve / discard
                                'depth' = 2, // numero de niveles que serán mostrados
                                'walker' = '' 

                            )
                        );

                    }
                        ?
                /nav

            /div
        /header

Functions.php, i want add the walker class but, I don't know what it's for that, if add the order menu or dont...

function yahcreatheme(){
        register_nav_menus(
            array(
                'primary' = __('Primary Menu', 'Yahcrea Theme'),
                'secondary' = __('Secondary Menu', 'Yahcrea Theme')
            )
        );
}

How can i add in my theme custom background for each page?