I downloaded a theme. The home page looks nothing like the themes though

What do i do?

You need to tell the system what theme you want to use. Update the pages.theme variable in your system.yaml (config docs ).

Could you give me steps? (Im new)

From the terminal, edit user/config/system.yaml and find

pages:
  theme: BLAH

and change BLAH to the name of the theme you just downloaded. If you're using the Admin plugin, then I can't help, I'm afraid.