Add CSS similar to the following in the editor that appears:
/* ".rli-sideBar .sidebar" selects the leftmost sidebar */
.rli-sideBar .sidebar {
/* Makes the sidebar have a white background. */
background-color: white;
}
/* Give everything in the sidebar a black foreground color. */
.rli-sideBar .sidebar * {
color: black;
}
/* Gives the currently selected notebook in the sidebar an orange background */
.rli-sideBar .sidebar div.selected > * {
background-color: orange;
}