The first and only thing you will need to do, to build your custom login page, is to have node and npm intalled on your computer.
Luckily, if you haven't, you can sort that out in a breeze by just following this quick guide: https://nodejs.org/en/download/package-manager/.
After that, clone this repo and on its root dir just type
npm install
This will tell npm to install all the needed dependencies required to build your custom login page.
When npm is done with it, just type
npm run build
to start the build process.
At some point you will be prompted to a small CLI that will ask you the settings for your custom login page.
Defaults will be presented between parenthesis and empty answers will assume the default values.
Once this is done, an output folder will be created with your generated artifacts.
Now you have a custom login page you can setup anywhere! Pretty sweet, hey? 😉
If you already know the settings you have on conf/login-page-content.json are the right ones, you can skip the CLI by typing instead
npm run build-nocomposer