Different color gap in between sections

I want my page to be this stone white: the header, main section, and footer are all styled with it but how can I fix this black gap? It comes and goes as my browser window size changes, is there a way to edit the color? Or get rid of it completely? I am obviously very beginner, thanks for the help.

Hi,

What is site url?

Or add to Design > Custom CSS

body {
	background-color: #f7f7ff;
}