Edit: Ignore my answer, misunderstood your question, can’t help with this unfortunately.
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I don’t know how to get them half/half but I’ve managed to get the background for menu.
1. You need a background image in size 960×81 px (it’s a guess but looks fine)
2. Upload your file (Media>Add New)
3. In customizit!>Custom CSS (or in your child theme if you’re using one) add the following lines:
/* navbar background */
.navbar .navbar-inner {
-webkit-box-shadow: 0px 0px 0px;
-moz-box-shadow: 0px 0px 0px;
box-shadow: 0px 0px 0px;
background-image:url('LINK TO YOUR FILE');
}
Replace “LINK TO YOUR FILE” with the actual link as shown in Media>Library
This background will not be displayed in mobile view.
I hope that someone more experienced can expand that and make the header size 50/50.