functions.php

<?php
/**
 * Sage includes
 *
 * The $sage_includes array determines the code library included in your theme.
 * Add or remove files to the array as needed. Supports child theme overrides.
 *
 * Please note that missing files will produce a fatal error.
 *
 * @link https://github.com/roots/sage/pull/1042
 */
$sage_includes = [
  'lib/assets.php',    // Scripts and stylesheets
  'lib/extras.php',    // Custom functions
  'lib/setup.php',     // Theme setup
  'lib/titles.php',    // Page titles
  'lib/wrapper.php',   // Theme wrapper class
  'lib/customizer.php' // Theme customizer
];

foreach ($sage_includes as $file) {
  if (!$filepath = locate_template($file)) {
    trigger_error(sprintf(__('Error locating %s for inclusion', 'sage'), $file), E_USER_ERROR);
  }

  require_once $filepath;
}
unset($file, $filepath);

$header = array(
  'width' => 1500,
  'height' => 890,
  'default-image' => get_template_directory_uri() . '/dist/images/header.jpg',
);

add_theme_support('custom-header', $header);

// Post excerpt length
function custom_excerpt_length( $length ) {
  return 35;
}
add_filter('excerpt_length', 'custom_excerpt_length');

// Post excerpt ellipsis
function new_excerpt_more( $more ) {
  return ' [...]';
}
add_filter('excerpt_more', 'new_excerpt_more');

// Menu items slug class
function add_slug_class_to_menu_item($output){
  $ps = get_option('permalink_structure');
  if(!empty($ps)){
    $idstr = preg_match_all('/<li id="menu-item-(\d+)/', $output, $matches);
    foreach($matches[1] as $mid){
      $id = get_post_meta($mid, '_menu_item_object_id', true);
      $slug = basename(get_permalink($id));
      $output = preg_replace('/menu-item-'.$mid.'">/', 'menu-item-'.$mid.' menu-item-'.$slug.'">', $output, 1);
    }
  }
  return $output;
}
add_filter('wp_nav_menu', 'add_slug_class_to_menu_item');

add_image_size( 'custom-thumb', 640, 360, array( 'center', 'center') );
add_image_size( 'custom-post-thumb', 300, 200, array( 'center', 'center') );
add_image_size( 'custom-splash', 1920, 1080, array( 'center', 'center') );

add_filter('post_gallery', 'my_post_gallery', 10, 2);
function my_post_gallery($output, $attr) {
    global $post;

    if (isset($attr['orderby'])) {
        $attr['orderby'] = sanitize_sql_orderby($attr['orderby']);
        if (!$attr['orderby'])
            unset($attr['orderby']);
    }

    extract(shortcode_atts(array(
        'order' => 'ASC',
        'orderby' => 'menu_order ID',
        'id' => $post->ID,
        'itemtag' => 'dl',
        'icontag' => 'dt',
        'captiontag' => 'dd',
        'columns' => 3,
        'size' => 'thumbnail',
        'include' => '',
        'exclude' => ''
    ), $attr));

    $id = intval($id);
    if ('RAND' == $order) $orderby = 'none';

    if (!empty($include)) {
        $include = preg_replace('/[^0-9,]+/', '', $include);
        $_attachments = get_posts(array('include' => $include, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby));

        $attachments = array();
        foreach ($_attachments as $key => $val) {
            $attachments[$val->ID] = $_attachments[$key];
        }
    }

    if (empty($attachments)) return '';

    // Here's your actual output, you may customize it to your need
    $output = "<div class=\"gallery clearfix\">\n";

    // Now you loop through each attachment
    foreach ($attachments as $id => $attachment) {
        // Fetch the thumbnail (or full image, it's up to you)
        $imgmed = wp_get_attachment_image_src($id, 'custom-post-thumb');
//      $img = wp_get_attachment_image_src($id, 'my-custom-image-size');
        $img = wp_get_attachment_image_src($id, 'full');

        $output .= "<figure class=\"gallery-item\">\n";
        $output .= "<a href=\"{$img[0]}\">";
        $output .= "<img src=\"{$imgmed[0]}\" class=\"gallery-image\" alt=\"Gallery Image\">";
        $output .= "</a>\n";
        $output .= "</figure>\n";
    }

    $output .= "</div>\n";

    return $output;
}