Hello,

It all depends on how your skin is designed. - Generally though you will be able to set this within your skin.css file.

You could try changing the body class - ie:

body {
background-color: #333;
}

But you may find that if you have designed the skin using tables that this will not have an effect. - You then need to have a look at any classes assigned to your tables and see if you can change the background color for those classes. - To test this out you could use:

body, tr, td {
background-color: #333;
}

You may find you have assigned a CSS class such as this below to one of your main tables:
.skinmaster {
    background-color: #333;   
}

Hope this gives you a starting point, thanks,