Background To White in Body and Posts

I am working off of an Adelle child theme in www.ads-software.com and want to make the middle body of the page white with a background image showing on the sides. I have searched and looked for solutions but nothing seems to work. I have added various css codes to make a white wrapper or background over the image that I have found in forums as possible solutions to my style.css in my child theme but nothing changes. my site is https://sarahfinadesigns.com/ and I am just starting to customize it (imported posts from blogger). Any advice is appreciated. The look I am going for is like this site that uses the same parent theme of adelle: https://jayjayne.com/. I see that it is on wordpress.com but I am hoping there is a customization option for my self hosted site?

Thank you.