HAI i know this is really late but i still wanted to help 

THIS ISNT MY CODE!!!!! i found a layout with a header and i took the code for the header from that and used itttt. i used yaoilicious' layout as da source sodfohdfuhwg
ALSO this is JUST for images!!!! i havent found a post for changing its color, but you can just use an image of the color you want here.
code:
<style>
nav .top {
background: transparent;
padding: 15px 10px 14px 10px;
position: relative;
}
nav .links {
background-color: transparent;
padding: 3.5px 16px;
text-align: center;
font-size: smaller;
}
nav{
background: url(INSERT IMG URL HERE); n background-position-x: center;
background-position-y: -66px;
padding-bottom: 21px;
margin-top: 20px;
border-radius: 10px;
border: 1px solid white;
margin-bottom: 20px;
box-shadow: 0 0 5px white, 0 0 10px white, 0 0 15px white;
font-size: revert;
}
</style>
of course since i took the code from another premade layout there are extra things like the box shadow n stuff (idk) you can remove those but i kept them on here just incase u needed it !!! :3
im pretty sure if you already have a premade layout with a header you wont be able to use this, you need to look for the header code in the layout and manually change it!!!
i am not 100% sure this works with everyone but it did for me sooyeahhhdhsdf