Hello,

To add a shadow around the main content area and the sidebars of your website please paste in the following custom CSS code to your Admin Dashboard > Appearance > Customize > Theme Options > Advanced Options > Custom CSS field :

/* Shadow For Primary Sidebar */

#sidebar-primary .area-wrapper {
    box-shadow: 0 0 48px #000;
}

/* Shadow For Secondary Sidebar */

#sidebar-secondary .area-wrapper {
    box-shadow: 0 0 48px #000;
}

/* For Main Content Area */

main#main {
    box-shadow: 0 0 48px #000;
}

You can customize the CSS code to increase / decrease the amount of shadow (px) and also change the shadow color to the hex color code of your desired color.

Hope this Helps,

Best Regards !!