What knowledge would be requireed to customize the pages and look in wordpress

I w’d like to edit customize the sign in and sign up pages of my multisite.It should affect my main site as well as children sites. I am trying to add do it by HTML .But it is giving errors .

I am doing the same to edit the plugins look too.The same error .

I am not an expert in PHP .How can I customize these pages in wordpress?

Thank you !