How do edit my code to show solid box colour to my navigation with CSS code?

Hi,

How do I edit this code to show a solid colour box on the navigation on the left? When scrolling, there is an overlap of elements (like the divider) with the navigation on the left.

Website: https://sunflower-parrot-b669.squarespace.com/

pass: demo

Thanks in advance

Code:

/* Float header */
@media screen and (min-width:992px) {
  .header-title {
    position: fixed;
    top: 3vw;
    left: 2vw;
    z-index: 9999;
}
.header-nav {
    position: fixed;
    top: 10vw;
    left: 2vw;
    padding: 0 !important;
}
  nav {
    flex-direction: column;
  }
  div.header-nav-item {
    margin: 0 !important;
    text-align: left !important;
}
  div.header-nav-item  a{
   display: inline-block;
}
.header-actions.header-actions--right {
    position: fixed;
    left: 2vw;
    bottom: 2vw;
    justify-content: flex-start !important;
}
.header-actions--right .header-actions-action a {
    margin-left: 0 !important;
    margin-right: 2.5vw;
}
#page article section {
    max-width: 85%;
    margin-left: 15%;
    padding-top: 0 !important;
}
  .header-title-text {
    text-align: left;
}
  header#header {
    box-shadow: none !important;
}}

Change this code

to this

#page article section {
    max-width: 85%;
    margin-left: 15%;
    padding-top: 0 !important;
	overflow: hidden;
}