fix: Swapping purple and green colors for gift selection states

A fix for the change described in this ticket - #26 - Donuts noted that the purple and green states used for gift selection were confusing, as green looks more like a "selected" state than purple but was being used to indicate "unselected." These color assignations have been reversed by updating the relevant SASS definitions for tertiary button elements, and for .Perk-imgButton's border color when the related gift form element is in the selected state.

Closes: #26