Customise ionic.css from lib\ionic\css

Can anyone suggest how should I go about as I would like to customise below class, what is the best way

  1. Shall I customise directly on lib\ionic\css\ionic.css?
    or create a new .css?

Original
.item {
overflow:hidden;
text-overflow:ellipsis;
white-space:nowrap
}

edit to
.item {
overflow:visible;
text-overflow:—;
white-space:nowrap
}