If you want to customize templates with existing theme, that supports remote updates, the best practice is to start with creating a child theme, that would take existing theme ( in this case - Corporate Blue ) as parent theme.

This way, you avoid possible overwrites during theme updates, and also allows you to override template files that exist in parent theme, by placing templates files of same name in child theme directory.

There are few steps to achieve this:

1. Create child theme
2. Clone/Override or create page template files in child theme
3. Clone/Override or create template files in child theme for post types single views, post type archives, taxonomy, front page, etc.

1. Child Theme

To create child theme, you need to create at least style.css within child theme directory. In your case, it would look something like this:

/*
Theme Name:   Corporate Blue Child
Theme URI:    http://themeurl.com/your-theme-slug/
Description:  Corporate Blue Child Theme
Author:       Your Name
Author URI:   http://example.com
Template:     corporate-blue
Version:      1.0.0
License:      GNU General Public License v2 or later
License URI:  http://www.gnu.org/licenses/gpl-2.0.html
Tags:         tags, that, describe, your, theme
Text Domain:  corporate-blue-child
*/

This would tell WordPress that this theme depends on Corporate Blue theme, and to look up for template files in parent theme, if template files of same name don\'t exist in child theme.

However, this wouldn\'t include parent theme styles.

To include parent theme\'s css, assuming that main stylesheet file is style.css,

create a functions.php file ( which WordPress loads automatically ), and place this code:

The key part here is get_template_directory_uri(), that tells WordPress to basic directory to look up for file is parent theme directory. And this function looks up for Template tag in style.css of child theme.

For more details see:

https://developer.wordpress.org/themes/advanced-topics/child-themes/

Keep in mind that child theme should be activated, not parent theme.

2. Custom page template files in child theme

After that you need to figure out what you need of custom pages.

Easiest way to create custom page template is to clone page.php or index.php, and rename it.

In the top of the cloned file, place php comment with Template Name:

<?php /* Template Name: Example Template */ ?>

That would allow WordPress to list all page templates for global use when adding/editing page. It would show them in drop down, usually on right sidebar when adding/editing page.

There are also page templates for specific pages. If you know page ID or page slug, you can add either one as suffix to template name:

page-{slug}.php
page-{ID}.php 

This way, WordPress would recognize and load such template for specific page.

See more about custom page templates:

https://developer.wordpress.org/themes/template-files-section/page-template-files/

3. Template files in child theme

Templates for post types, archives, category pages, front page, blog archive page, etc.

You need to learn more about template hierarchy:
https://developer.wordpress.org/themes/basics/template-hierarchy/

Yes, we can change the wordpress theme\'s files, but don\'t do this if you don\'t have any knowledge of wordpress structure and php. It might break your theme.

If you only want to create custom pages without changing the functionality, here is the guide you should follow.

https://www.wpbeginner.com/wp-themes/how-to-create-a-custom-page-in-wordpress/

It will help you to create the custom pages in wordpress.