You mean the background color of the currently selected notebook, right?
I may be mistaken, but I couldn’t find a way to do this via CSS. It seems like it would be styling list-item-container, but the background color for that item is being applied via element styling vs a class.
Div of currently selected notebook
<div class="list-item-container" background-color: rgb(1, 63, 116);" draggable="true" folderid="x..." style="box-sizing: border-box; height: 25px; display: flex; align-items: stretch; >
Div of Non-selected notebook
<div class="list-item-container" background-color: rgba(200, 200, 200, 0);" draggable="true" folderid="y..." style="box-sizing: border-box; height: 25px; display: flex; align-items: stretch;>
Ideally the first element would have a class="selected" that was changing the background color, or even just there as a styling hook.
Curious if anyone else has found a work around.