Log in to your dashboard--> layout- -> Edit HTML, scroll to where you see this:-

    #header-wrapper {


Add immediately after that the following line:-

    background:your clour;


If You want background colour set to #D3FFCC , above code must change like this.
background:#D3FFCC;

See the example below.

    #header-wrapper {
    background:#D3FFCC;


Save the template and refresh your site.