If you use a banner you can add css to the editor in admin>customize>CSS/HTML. Something like this will give you a banner with a white background that only displays on the categories-list page:
#banner {
max-height: none; // gets rid of scrollbar
display: none; // hides the banner for all pages
background: #fff; // gets rid of the blue background
box-shadow: none; // gets rid of the box-shadow
}
.categories-list #banner {
display: block; // displays the banner only on the categories-list page
}
#banner .close {
display: none; // hides the dismiss 'x'
}
#banner h1 {
margin: 30px 0 10px 0; // gives the banner h1 elements top and bottom margins
}