Color Margin

file: https://www.aas-bd.com/wp-content/themes/twentyelvenchildtheme/style.css
line: 119

#page {
    background-color: #D3D6D7;
    margin: 0 auto;
    max-width: 1024px;
}

change the auto to 8% or 100px or 1.2em
as for the color of the margins,
you can change this in your admin – appearance – theme setting / background

By keeping auto is it possible to change the background margin color?

background margin color does not exist.
use:

body {
    background-color: blue;
}

or what ever color you want.
HTH

Actually I mean page margin color. I changed it into #4012a2; from theme option. but it does not work. Still it looks like white.

margin color does not exist
Your page has this in its header:

<style type="text/css" id="custom-background-css">
body.custom-background { background-color: #f1f1f1; }
</style>

so that’s either your theme or a plugin.
If you are editing the css in your theme settings page, found at appearance – background / theme options if there is one.
This would be added by a plugin.

If you have a back up of your wp-content folder on a local machine, you could do an in file search for “custom-background-css” if you don’t know where this style is being added from.

HTH

Aasbd, you can ask as many CSS questions as you like in CSS forums such as;
https://csscreator.com/forum
https://www.cssforum.com.pk/
https://css-tricks.com/forums/
https://www.welovecss.com/
https://www.codingforums.com/

www.remarpro.com forums are meant to be for www.remarpro.com support.