I am going to assume the Passage content of your example looks something like the following:
End of Text.
[[Choice 1|Next]]
[[Choice 2|Next]]
.. if so then I suggest you wrap the choice links in a div element that has an ID, the following example uses Custom Style based markup to do that and assigns the resulting div an ID of choices.
End of Text.
@@#choices;
[[Choice 1|Next]]
[[Choice 2|Next]]
@@
You can then use CSS like the following within your Story Stylesheet area to style the new choices div element like so:
#choices {
border: 4px solid red;
padding: 1em;
background-color: teal;
}
note: you may need to play around with the above values to get the exact look you want but they should act as a starting point.