To add custom capabilities to a user role –
1. Identify the Role:
Determine which user role you want to modify, for example, Administrator, Editor, Author etc.
Add custom capability to a Role:
Use the `add_cap` method to add capabilities. You can do this in your `functions.php` file of your theme or in a custom plugin.

function add_custom_capabilities() {
  $role = get_role('editor'); //Replace 'editor' with the role you want to modify
  if ($role) {
    $role->add_cap('custom_capability'); //Replace 'custom_capability' with the capability you want to add
  }
}
add_action('admin_init', 'add_custom_capabilities');

Remove Custom Capability from a Role:
Use the `remove_cap` method to remove capabilities.

function remove_custom_capabilities() {
  $role = get_role('editor'); //Replace 'editor' with the role you want to modify
  if ($role) {
    $role->remove_cap('custom_capability'); //Replace 'custom_capability' with the capability you want to remove
  }
}
add_action('admin_init', 'remove_custom_capabilities');

After making changes, test the role to ensure it has the correct capabilities and restrictions.