Set your header wrapper and header to the attributes you see below:
#header-wrapper {
width: 800px;
margin:0;
padding: 0;
background-color: #336699;
text-align: center;
}
#header {
width: 800px;
margin: 0 auto;
background-color: #336699;
border: 1px solid #336699;
color: #ffffff;
padding: 0;
font: normal normal 210% Verdana, Arial, Sans-serif;;
}
You can also find a post on adding a header image here.
The method described in this post will give you more control of the image than what you get with Blogger's page element method.
Hope this helps!
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