How can I split the background color on the Enterprise theme header?

[Linda_Ladley]

We're trialling the option of updating our theme, can anyone advise how to split the background colour on the Header please?

We'd like to retain Navy for the logo / search bar etc... and then a white background for the menu but it looks like it's all one block. 

https://vrgrml3uvrodkkiq-17838313.shopifypreview.com

[Asad24]

Go to your online store -> Customize -> Settings -> Custom Css and paste this code in the custom css section

.main-nav, .secondary-nav {
color: black !important;
background-color: #ffffff !important;

}

[Linda_Ladley]

Thank you - we had tried that earlier but we end up with the following - the menu blocks are white - but it doesn't fill the row and the menu drop down is still blue.

[Linda_Ladley]

The preview link expired - https://pmvm3z8k9s7rhnh3-17838313.shopifypreview.com

Can anyone advise if you can split the colours so the top half of the header (logo and search) is navy and the menu is white? 

The previous suggestion didn't quite make it work.