Hi Christopher,
What is happening is that your CSS class is less specific than the one of OutSystems UI.
You are using .desktop .list-item-selected whereas OutSystemsUI is using .desktop .list .list-item-selected.
Can you try using the code below (matching the classes specifity of OutSystems UI so your theme would override OutSystems UI's):
.desktop .list .list-item-selected,
.tablet.landscape .list .list-item-selected {
background-color: var(--color-primary-lightest);
color: var(--color-neutral-0);
}
Hope it helps.
Cheers,
João