How do I add a custom page id so I can make individual customizations?

Please follow the steps:

  • Step 1: Go to Online store > Themes > Actions > Edit code.
  • Step 2: Go to layout > theme.liquid and add code: https://i.imgur.com/FIdZ4xt.png
  • Step 3: You just need to turn on Inspect of the browser to check, at each page it will display a different class for the body.
    Ex: https://i.imgur.com/KwA3IPg.png for login page.
    Hope it clear to you.