You can use these apps (search apps in odoo apps by names provided below) to change the whole login page style.

Moreover, these apps provide the ability to define your own favicon and logo as you wish.

Apps Name:

Minimalist Login Form

Creative Login Form

Advanced Login Form

Niyas answer is correct plus that you can do so using Odoo Studio in Enterprise version without coding

Hi,

There is a free module available for the same in odoo apps, please have a look at this module: Odoo Web Login Screen

Thanks