Hi, concepta.
Unfortunately what you want to do isn’t possible with the Razor theme. The white background colour is behind the entire #MiddleInner
container. You can make that transparent…
#MiddleInner {background-color: transparent !important }
…but then the 3/4 area is also transparent. You can make that specific element have a white background…
#header-general-layout-test_c2_col-3-4_2 {
background-color: white !important
}
…but then you have a problem with padding and margins.
With the Razor theme I think it is going to be very difficult for you to get the look you are after. I think you would need to customise quite a lot of the theme’s CSS, and even then I think some of the PHP that generates the HTML would also have to be customised.