How to change color of margin padding in Jasper template?

Site URL: https://www.markhilton.me

Hello,

I am curious as how to change the color of the margin padding on my site which uses the Jasper template. I can't seem to find the option in Design > Site styles. I would like to make the margin padding a light grey for focus the reader's eyes on the actual content and provide definition.

I have picked out a color to use with a hex code, so if someone could provide the appropriate CSS code, I would appreciate it.

Add to Home > Design > Custom CSS

main#page {
    background: red;
}
.main-content {
    background: white;
}