To hide the search box in the Sight theme header area and make the site title and description area full width, add this to your Appearance → Custom Design → CSS editor:
.header-nav-search-container {
display: none;
}
.site-header hgroup {
width: 100%;
}
If you would like the header image to be wider, find this CSS block in your Appearance → Custom Design → CSS editor, and replace the url() value with a wider image:
.site-header hgroup {
padding-bottom: 174px;
background:url('http://user.chol.com/~h92002/background2.jpg');
background-repeat:no-repeat;
background-position:left bottom
}