If it is always going to be the last tab, then you can use the following:

.cf-pagination-tabs li:last-child {
  background-color: red;
}

However, you could also use the order:

.cf-pagination-tabs li:nth-child(4) {
  background-color: red;
}

If you have pages that show/hide based on user selections that could change the ordering, you can right-click the tab in the browser to inspect the page and find the "page" attribute value (this corresponds to the ID of the actual page), and then you could reference it by that as well.

.cf-pagination-tabs li[page="q161"] {
  background-color: red;
}