Color Theme Questions

Hello, I’m new here and have a few general questions about themes:

  • (How) is it possible to change the background color of the page with theme variables? I want every new page I create to be initialized with the color of a theme variable as background color.
  • (How) is it possible to change the color of the keyboard?
  • (How) is it possible to change the color of the system topbar (ios, android)?
  • Is it somehow possible
  • When I test input fields on my iOS phone and use more than one input field, a strange autocorrect occurs. I would like to better understand when this occurs and if it can be prevented.

Another general question about Firebase authentication:
I want to prepend an onboarding page to my login/registration page, but can’t find a good way to do it.

Thanks for any help.