Thanks for writing in!
For the navigation, you can make use of the
menu-name-of-the-menu as the ID. Please make sure that you replace the 'name-of-the-menu' exactly the same as the way you named your menu in Appearance > Menus.
For the main content, you will have to make a custom ID for it. Because what you are trying to accomplish requires a template customization, we would highly to suggest that you use a child theme. This allows you to make code changes that won’t be overwritten when an X update is released. After your child theme is setup, please review how we recommend making template changes in Customization Best Practices.
After the child theme is set up, please add the following code in your child theme’s functions.php file
echo '<div id="my-main-content"></div>';
And you can make use of the
my-main-content as the ID for your content.
Hope this helps. Please let us know how it goes.