.mybanner {
  min-width: 100%;
}
.container-header {
    background-color: #FFFFFF;
    background-image: none;
}

.container-header .mod-menu {
    colour: #silver;
}
.container-header .mod-menu a {
  -webkit-border-radius: 0 0 6px 6px;
  -moz-border-radius: 0 0 6px 6px;
  border-radius: 0 0 6px 6px; 
  padding: 10px 20px;
  colour: #FFFFFF;
}

.container-header .mod-menu a:hover {
    background-color:#ff0000;
}
.container-header .navbar-toggler {
    color: #ff0000;
    border: 1px solid #0088cc;
}
In the following, I will summarise the code in the file user.css.

/* Banner Image */
.mybanner {
  min-width: 100%;
}

/* Menu */

/* Override the lila background with white */
.container-header {
    background-color: #FF0000;
    background-image: none;
}

/* Text should now be blue */
.container-header .mod-menu {
    color: #FF0000;
}

/* We add padding and radius so that on hover there is a nice background */
.container-header .mod-menu a {
    -webkit-border-radius: 0 0 6px 6px;
    -moz-border-radius: 0 0 6px 6px;
    border-radius: 0 0 6px 6px; 
    padding: 3px 15px;
    color: #ff0000;
}

/* On hover there should be a gray background*/
.container-header .mod-menu a:hover {
    background-color: #A9A9A9;
}

/* We need to change the color of the Hamburger Menu because white on white is not good */
.container-header .navbar-toggler {
    color: #0088cc;
    border: 1px solid #0088cc;
}
.site-grid {
    background-color: white;
}
@media (max-width: 991px) {
    .site-grid {
        grid-template-areas:
            ". banner banner banner banner ."
            ". top-a  top-a  top-a  top-a  ."
            ". top-b  top-b  top-b  top-b  ."
            ". side-r side-r side-r side-r ."
            ". comp   comp   comp   comp   ."
            ". side-l side-l side-l side-l ."
            ". bot-a  bot-a  bot-a  bot-a  ."
            ". bot-b  bot-b  bot-b  bot-b  ."
    }
}
.header {
    background-color: white;
}

.grid-child {
    padding: 3px 15px;
    background-color: white;
}

.footer {
    background-color: white;
    background-image: none;
}