Thank you! It worked perfectly. Instead of using the shortcode, I wrote out the html for the buttons and added in the custom class. The only issue I had afterward was that the buttons no longer showed the hand mouseover effect – but I fixed that by adding cursor: pointer; to the style.
They looked like this in the end:
<a class="nectar-button small accent-color regular-button sg-popup-id-1" style="visibility: visible; cursor: pointer;" target="_blank" data-color-override="false" data-hover-color-override="false" data-hover-text-color-override="#fff">Click here</a>
