Popup CSS DIV ID keeps changing.

I made a popup on one of our sites and then I created some custom styles and attached them to the DIV ID.

For example, this ID worked a while ago
div#a8e296a067a37563370ded05f5a3bf3ec-po div.claimbutton { bla bla bla }

But after a couple of days the ID automatically changed to a9bf31c7ff062936a96d3c8bd1f8f2ff3-po

This is a problem as every time the layout breaks, please advice.