Which files control what content?

I’m new to css, and know practically nothing about php. I’m trying to get the wp blog formatted properly at the beta site https://www.declaringglory.com/blog

I want the look to be more like https://declaringglory.com/nihc_homebuyers.html

What files control the blog content? How do I get the posts and archives etc to behave? how do I get that white background back/and bottom bar contained to the width of the top?

If somebody is willing to walk this through with me, I can share the ftp info offlist to see the actual files and css.