Create an new admin theme

In the Bagisto root directory go to the config folder and open themes.php.
Now search for the keys admin-default and admin-themes. It will look like this,

<?php
  return [
    'admin-default' => 'default',

    'admin-themes' => [
      'default' => [
        'views_path' => 'resources/admin-themes/default/views',
        'assets_path' => 'public/admin-themes/default/assets',
        'name' => 'Default'
      ]
    ]
  ];

Key	Description
admin-default	is used to set the current theme.
admin-themes	is used to store a list of themes

Now add one more theme into admin-themes.

<?php
  return [
    'admin-default' => 'default',
    
    'admin-themes' => [
      'default' => [
        'views_path' => 'resources/admin-themes/default/views',
        'assets_path' => 'public/admin-themes/default/assets',
        'name' => 'Default'
      ],
        
      'theme1' => [
        'views_path' => 'resources/admin-themes/theme1/views',
        'assets_path' => 'public/admin-themes/theme1/assets',
        'name' => 'Theme 1'
      ]
    ]
  ];

As you see we have added the view and assets path in the theme1 array.
Now set your views and assets folder according to your mentioned path.

The resources structure will look like this,

    - resources
      - admin-themes
        - theme1
          - views

Assets structure will look like this,

    - public
      - admin-themes
        - theme1
          - assets

Now lets replace the dashboard page.
Just create the exact dashboard's directory structure for theme1 as present in default.
So it will look like this,

    - resources
      - admin-themes
        - theme1
          - views
            - dashboard
                - index.blade.php

Add some lines to the index.blade.php,

    Theme 1 Sample

Now everything is setup. Just one thing is pending i.e. we need to activate the theme. So, for that we need to change the admin-default key in file config/themes.php.

    ...
    'admin-default' => 'theme1',
    ...

Now check the dashboard page.