The start page is always the first page below the website and you can’t change that.
If you follow the Developer introduction (litium.com) you will have your own installation up and running of the Mvc Accelerator. This installation will include a login page, and more, and you will have full access to the source code so you can investigate how the login is built.
When it come to the React Accelerator the login page it’s not included yet and will be in a later version.