You can do this just by adding :
#main-content {
background-color: blue!important;

Here you force the main content to be blue instead of white for instance (will work for page/post/projects) but sections have their background set to white by default, so when adding a section, you’ll have to select a color for background or activate “transparent background” to see the color you selected.

Why do section have a default background color… Just because it allows you to mix different colors on a same page i guess.